Issue in Honor of Prof. Albert Padwa ARKIVOC 2002 (viii) 157-165
The stereochemistry of addition of allyl sulfone carbanions to aldehydes. Formation of dihydrofurans 1
Alfred Hassner,* Avital Laxer, and Eugene Ghera
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
E-mail:
[email protected]
This paper is dedicated in friendship and with best wishes to Prof. Albert Padwa on the occasion of his 65th birthday (received 17 Jun 02; accepted 10 Oct 02; published on the web 18 Oct 02)
Abstract The reaction of the mono-anion of the bromoallyl sulfone 1 with aldehydes 2 was examined with the aim of obtaining selectively substituted tetrahydrofurans. At –100ºC syn- and anti- open chain adducts 3 and 4 were isolated together with a low yield of 4-methylene-2,3-disubstituted tetrahydrofuran 5. In the presence of HMPA or at higher temperature the reaction led to formation of 2,5-dihydrofurans 6. The stereochemical results are consistent with initial addition of 1 to the aldehyde involving Li ion chelation.
Keywords: Dihydrofurans, Michael additions, allyl sulfones, Li chelation
Introduction
Recently we have shown 2,3 that 2-(bromomethyl)-3-phenylsulfonyl-1-propene 1 reacts with one molar equivalent of LDA to generate a lithiated a-allylsulfone carbanion that is stable at low temperature and undergoes regioselective and stereoselective additions via the a-carbon to Michael acceptors such as unsaturated esters, ketones, sulfones and nitro compounds, followed by cyclization to methylene-cyclopentenes. For instance, in the case of unsaturated esters, the primary Michael adduct was not even isolable at –78ºC and the incipient carbanion immediately underwent intramolecular reaction with the allylic bromide to afford stereoselectively substituted methylenecyclopentane derivatives (eq. 1).2 The high stereoselectivity in the product can be rationalized on the basis of Li-ion chelation by the sulfone and the ester function during addition.
The addition of lithiated 1 to the C=N of sulfinimines proceeded less stereoselectively and led with double bond rearrangement to 2-arylpyrrolines.3c
ISSN 1424-6376 Page 157 ©ARKAT USA, Inc
