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58
YflK 547.314
BecTHHK Cn6ry. Cep. 4. 2013. Bun. 4
E. Yu. Schmidt
NEW HORIZONS OF THE FAVORSKY CHEMISTRY*
The Readings devoted to the memory of academician A. F. Favorsky, the outstanding chemist of the modern times and the founder of one of the largest chemical schools, have been held in A. F. Favorsky Irkutsk institute of chemistry, Siberian Branch, Russian Academy of Science (RAS) in the beginning of this year (February, 18-20, 2013).
The Irkutsk institute of chemistry was founded as a part of the Siberian Branch of the USSR Academy of Sciences in 1957 by the corresponding member of RAS M. F. Shos-takovsky, closest disciple of A. F. Favorsky. For many years, the institute was headed by academician M. G. Voronkov, the last PhD student of A. F. Favorsky. Traditionally, the institute is engaged in the development of acetylene chemistry, one of the founders of which is recognized to be academician A. F. Favorsky. In 2000, for the successes reached in the field of acetylene chemistry the institute was named after academician A. F. Favorsky by the Decision of Presidium of RAS.
The first Irkutsk Readings dedicated to the memory of A. F. Favorsky were organized as a conference, the program of which included plenary lectures on urgent problems of organic chemistry, first of all, acetylene chemistry. The second part of the conference comprised the reports made by the heads of the institute laboratories on their major achievements over the past year.
The participants of the conference have noted that deep and systematic investigations of academician A. F. Favorsky scientific heritage open the new horizons in organic synthesis. The present paper convincingly illustrates this conclusion.
The Favorsky reaction (or Favorsky ethynylation reaction or Favorsky alkynol synthesis according) [1] is the addition of acetylenes to carbonyl compounds in the presence of the strong bases to afford acetylenic alcohols. Such direct formation of the Csp3—Csp bond occurs under exclusively simple experimental conditions and allows introducing the acetylenic fragment almost into any organic substrate containing the carbonyl group in the presence of such available basis as potassium hydroxide. A role of the Favorsky reaction in fine organic synthesis can hardly be overestimated: acetylene ethers [2-4] unsaturated ketones [5-7], isoprenoids, steroids [8], heterocyclic compounds [9, 10] have been synthesized using acetylenic alcohols. The Favorsky ethynylation reaction provides the basis for industrial synthesis of isoprene [11], carotinoids [12], vitamins A and E [11, 13], fragrance compounds [14], herbicides [11], corrosion inhibitors [11], nonionic surfactants [11].
Systematic application of the superbase systems such as, for example, potassium hydrox-ide/hexametapol [15] or potassium hydroxide/dimethylsulfoxide [16, 17] has given qualitative impetus to the development of the Favorsky reaction. In such systems, aldehydes and ketones adds acetylene much more actively than under traditional conditions. Consequently, diverse acetylenic alcohols can be synthesized in high yields without pressure, without cooling, without large amounts of anhydrous alkalis and without large volumes of solvents, mandatory attributes of classical ethynylation.
Elena Yu. Schmidt — Professor, Favorsky Irkutsk Institute of chemistry SB RAS; e-mail: [email protected]
* Plenary lecture of Dr. Sci. Elena Yu. Schmidt, A. E. Favorsky Institute of Chemistry, SB RAS "Reactions of acetylenes with ketons. Is the Favorsky synthesis only?".
© E.Yu. Schmidt, 2013
Tertiary acetylenic alcohols, formed by ethynylation of ketones with acetylenes, dissociate into initial ketones and acetylenes at increased temperature in the presence of strong bases. This reaction is known in organic synthesis as classical reverse Favorsky reaction [18].
At first glance these two fundamental reactions (direct and reverse Favorsky reactions) allowed no possibility for the realization of any other base-catalyzed reactions of ketones with acetylenes, and novel findings in this are seemed to be improbable.
However ketones as CH-acids are deprotonated under the action of strong bases (especially at increased temperatures) and, hence the formed carbanions (or a-carbonyl carban-ions) can be added to the triple bond of acetylene.
Indeed, one has managed be made this probability real. It turns out that ketones at 80-100 C in the superbase systems like hydroxide (alkoxide) alkali metal/DMSO are capable of adding to arylacetylenes in C-nucleophilic manner [19-24]. The reaction is regio- and stereoselective, the products being P,y-ethylenic ketones of ^-configuration.
R2 R2
VMOR/DMSO t,i I 80-100 C, 30-60 min |f
O O
R1, R2 = Alk, cyclo-Alk, HetAr; R3 = Ar, HetAr; M = Na, K, Cs; R = H, *Bu.
The reaction tolerates aliphatic, cycloaliphatic, alkylaryl- and alkylhetarylketones, i. e. any ketones bearing no functional groups sensitive to the strong bases. The reaction well "works" in a series of aryl- and hetarylacetylenes.
The discovery of this reaction (vinylation of ketones with acetylenes) essentially expands the traditional representations about reactivity of two such large and important classes of organic compounds as ketones and acetylenes.
The potential of this reaction only starts to unveil. Recently, we have reported on one-pot synthesis of A2-isoxazolines from ketones, arylacetylenes and hydroxylamine [25]. The ketone is first treated with arylacetylene in the system potassium tert-butoxide/DMSO (100 °C, 30 min) and then with hydroxylamine hydrochloride and potassium hydroxide (70 °C, 30 min). Naturally, P,y-ethylenic ketones, participating in the assembly without preliminary isolation, represent key intermediates of the reaction.
2 1. KO'Bu/DMSO
R2 2. NH2OH HCI R1 R2
V R -' N^R'
OO
R1, R2 = Alk, cyclo -Alk, Ar; R3 = Ar.
A wide series of isoxazolines with different substituents, easy variation of which is provided due to a diversity of both ketones and arylacetylenes, has been synthesized by this reaction. A2-Isoxazolines represent powerful pharmacophores [26-28], in particular tuberculostatic agents [29, 30].
Cyclohexanones react with arylacetylenes in the KOH/DMSO suspension to stereose-lectively furnish dispirocyclic ketals, which are assembled via one-pot procedure from two
molecules of ketones and one molecule of arylacetylene [31].
R1
+ E
KOH/DMSO ->
80 C, 1 h
O
R
R
R1 = H, 3-Me, 4-Me; R2 = H, 2,5-Me, 4-Ph, 3-F.
Dispirocyclic ketals combine in their molecule two essential structural motifs of the Fa-vorsky chemistry: vinyl ether and cyclic acetal (1,3-dioxolane fragment, which always accompanies the vinylation of 1,2-diols, or generated by acid-catalyzed cyclization of their monovinyl ethers). This synthesis may be of preparative importance as a one-pot approach to dispirocyclic ketals, congeners of renown antibacterial [32] and cytotoxic drugs [33].
Absolutely unexpected is the result of interaction between 2-alkylcycloalkanones with arylacetylenes in similar conditions: under the action of a strong base the reaction leads to diastereoselective assembly of hexahydroazulenones (in this case, from two molecules of arylacetylene and one molecule of ketones) [34].
,Ri + KOH/DMSO R + w R2 1 00 c , 1 h
R2
O
R1 = Me, Et, 4-Me; R2 = H, 4-Me, 4-Ph, 3-F.
Amazing reaction! Four carbon-carbon bonds are formed and the bicyclic system is produced. And all this occurs in one preparative step, from available reactants (ketones and arylacetylenes) and in the presence of such simple catalysts as potassium hydroxide. Azulenone compounds are abundant in nature (for example, they are contained in ether oils of daisy and milfoil) and attract great research attention due to their biological [35] and pharmaceutical [36, 37] importance. However, synthetic approaches to enanthio- and diastereoselective methods for the preparation of substituted azulenones are still limited by intramolecular cyclization of P-aryl-a-diazoketones (intramolecular Buchner reaction) in the presence of rhodium salts [38, 39] and cycloaddition reaction of 2-phenyl-2-acylketenes to acetylenic ethers [40].
With acetylene itself, cycloaliphatic ketones on heating and in the presence of a strong base behave in different manner: vinyl ethers of tertiary acetylenic alcohols are formed [41].
I ) „
+HC=CH<
KOH/DMSO
90 C, 1 h
Apparently, owing to reversibility of the Favorsky reaction, tertiary acetylenic alcohol, presented in the reaction mixture even in negligible concentration, are vinylated. These are the first examples of syntheses of hitherto unknown vinyl ethers of tertiary acetylenic alcohols [42].
Another absolutely unique reaction: alkylaryl(hetaryl)ketones and acetylene in the alkali metal hydroxide/dimethylsulfoxide suspension undergo a complex cascade of transformations resulted in the formation of 7-methylene-6,8-dioxabicyclo[3.2.1]octanes from two molecules of ketones and two molecules of acetylene [43]. In this case, the combination of vinyl ether and cyclic acetal (structural units typical for the Favorsky reaction), also take place.
R1 = Ar, HetAr; R2 = H, Alk; M = Na, K, Cs.
Dioxabicyclooctane skeleton is a basic structural motif of frontalin family insect pheromones [44-46], mammals behavior regulators [47, 48], plant toxins [49] and marine organisms [50, 51]. Currently, such compounds are synthesized via the sophisticated methods involving multi-step protocols. In our case, simple reactants (ketones and acetylene) in simple catalytic system (alkali/dimethylsulfoxide) are assembled in a one-pot fashion into complex bicyclic systems. Notably, the diastereoselectivity of the reaction reaches 100 % even when the molecules contain 5 asymmetric carbon atoms.
We are still closer to gain a more penetrating insight into this reaction. Basic intermediates have been isolated and characterized. Key step of the reaction involves the formation of 1,5-diketones, which further cyclize with acetylene into 7-methylene-6,8-dioxobicyc-lo[3.2.1]octanes [52] through the Favorsky ethynylation and vinylation reactions.
R1, R5 = Ar, HetAr; R2, R4 = H, Me; R1-R2 = cyclo-Alk; R3 = H, Me, Ph; R6 = H, Ar, HetAr.
The reaction found (cyclization of 1,5-diketones with acetylenes into 7-methylene-6,8-di-oxabicyclo[3.2.1]octanes) enables not only to construct the bicyclic backbone of dioxabicy-clooctane in a one preparative step, but also to simultaneously introduce diverse alkyl, aryl and hetaryl substituents into the different positions of the bicyclic core (in various combinations including bridge positions). Such possibility is ensured by accessibility of both acetylenes and 1,5-diketones (easily prepared by condensation of ketones with aldehydes or a,P-unsaturated ketones). Especially appealing is that stereoselective synthesis of bicyclooc-tanes is catalyzed by alkali metal hydroxides (NaOH, KOH), i. e. by compounds consisting of natural ions.
A fertile field of the Favorsky chemistry has yielded another reaction. If to introduce a third component, ketoxime, to the reaction of aliphatic ketones with acetylenes in the system lithium hydroxide/cesium fluoride/dimethylsulfoxide, complex heterocyclic systems, 4-methylene-3-oxa-1-azabicyclo[3.1.0]hexanes are formed [53]. These compounds contain
fragments of aziridine, oxazolidine and vinyl ether.
R3 R
1 ^ \ R2
Mev/R1 R\ J LiOH/CsF/DMSO N '
Y + v^ + hc^CH--—--► N
II 70-90 °C, 5-60 min . , \
OH R1
Oxaazabicyclohexanes are structural analogs of natural and synthetic immuno-modulators [54], antimicrobial agents [55], medications against glaucoma [56] and Alzheimer disease [57]. Both the reaction itself and combination of three synthetically and pharmaceutically important fragments in these heterocyclic systems pave the avenue for drug design.
A variety of reactions reported in this paper (and discovered only during the last 5 years) allows one to assume that a plenty of novel reactions related to the Favorsky chemistry will not take long to appear. Why these reactions remained unknown until the moment? Obviously because that before our investigations, the researchers did not systematcally employ the superbase system of alkali metal hydroxide (alkoxide) / dimethylsulfoxide. Such system activate both anions and acetylene due to classical and special effects of salvation-desolvation and, probably, owing to the complex formation with potassium cation which is supported by the results of quantum-chemical studies [58-60].
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Статья поступила в редакцию 16 апреля 2013 г.
