12 Umpolung of α,β-Ethylenic Ketones and Aldehydes by Phosphorus Groups
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H. J . CRISTAU, J . P. VORS, Y. BEZIAT, C. NIANGORAN, and H. CHRISTOL Laboratoire de Chimie Organique ENSCM, (Equipe de Recherche Associée au C N R S N° 610), 8, rue de l'Ecole Normale, 34075 Montpellier, France
Reactivity umpolung and synthetic equivalents are very fruitful concepts in modern organic synthesis, in particular for the multistep preparation of natural compounds. In this area, organophosphorus compounds can be useful tools, particularly for reversible umpolung by heteroatom exchange (1-4). Our objective was to prepare a phosphorus equivalent of β-a cylvinyl anion in order to obtain an anomalous nucleophilic reac tivity on the vinylic carbon β to the carbonyl group in α,β-ethylenic ketones and aldehydes. Examples of such umpolung (5-21), in cluding the use of phosphorus groups (22-27), are described in the literature. Except for a special case (25), a Wittig or a Wittig -Horner reaction is used to remove the phosphorus group, in such a way that the carbonyl compound, which functions as an oxidative alkylating reagent, is the only electrophilic counterpart to be used for the β-acylvinyl anion equivalent. We present here a reversible umpolung of α,β-ethylenic keto nes and aldehydes, in which the ylides 3 are synthetic equivalents of β-acylvinylanions in two unlike pathways (i) and (ii) (Scheme I, see next page). The preparation of the γ-thioacetalated phosphonium salts 2 takes place in a one-pot reaction by addition of triphenylphosphine hydrobromide on the α,β-unsaturated carbonyl compound and subsequent thioacetalation (28, 29) :
R
Η
1
Me
0
Me
0
- 85 %) with any phosphonium s a l t s we have tested. Subsequent e l i m i n a t i o n of triphenylphosphine by r e a c t i o n o f the s a l t s 5 with t r i e t h y l a m i ne was always nearly q u a n t i t a t i v e . 2
Seeomf pathway. With n o n - s t a b i l i z e d y l i d e s ( R = H o r Me) and numerous saturated or unsaturated aldehydes and ketones the W i t t i g r e a c t i o n ( gives the corresponding d i t h i a n e s 7 i n good y i e l d s (70-93 ί ) . The autoxidation o f these y l i d e s gives the expected products 8 and 9 . The s e m i - s t a b i l i z e d y l i d e s (R = 0 ) react only with a r e a c t i ve aldehyde. 1
1
R
1
c=o R=Η
R
1
R'*
ilh/Et 0/20°C 2
Η : 2Hh/glyme/reflux
R*=
Η,
R= Me, 2
(81 %) 0„/N„ 2 2
R*= Me,R Me
S^i
0
2
(53 %)
Me
In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
9
62
PHOSPHORUS CHEMISTRY
The l a s t steps o f the second pathway are w e l l known problems of organic chemistry which have s a t i s f a c t o r y s o l u t i o n s . Therefore we have not tested them. I n conclusion, the two pathways o f our umpolung method can be s a t i s f a c t o r i l y a p p l i e d to the α,β-ethylenic aldehydes or ketones bearing no s u b s t i t u e n t or an a l k y l a t the v i n y l i c carbon β to the carbonyl group ; but, when an a r y l group s u b s t i t u t e s t h i s p o s i t i o n , only the second pathway i s p o s s i b l e with some r e a c t i v e aldehydes.
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Literature Cited. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.
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RECEIVED
June 30, 1981.
In Phosphorus Chemistry; Quin, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
