title: Stereoselective conjugate addition of benzyl phenylsulfonyl carbanions to enoates derived from d-mannitol.
abstract: The conjugate addition of benzylic phenylsulfonyl carbanions (2a'-d') to enoates derived from d-(+)-mannitol (E- or Z-1a-c) was studied using THF and THF/HMPA as solvent. Under kinetic conditions (-78 degrees C), enoate E-1a,b led to a mixture of syn-(R,S) and anti-(S,S) adducts (55/45), and syn-(R,S) adducts were the main product obtained ( approximately 90/10) from enoate Z-1a. Under thermodynamic conditions (-78 degrees C to room temperature) syn-(R,S) adducts were also preferentially formed ( approximately 90/10), despite the geometry at the double bond in the acceptor. Enoate 1c (E/Z = 57/43), bearing an additional benzyl group at the alpha-position, also reacted with carbanions 2'a,b, under thermodynamic conditions, leading to syn-adducts in excellent de (control at the three newly generated stereogenic centers). The adducts were quantitatively transformed into the corresponding beta-gamma-disubstituted gamma-butyrolactones and alpha,beta,gamma-trisubstituted gamma-butyrolactones. (1)H NMR studies (NOE and J-coupling) of these lactones allowed us to determine their configuration at the newly generated chiral centers. The reduction of the C-S bond in adducts syn-(R,S) with Na/Hg, followed by treatment of the resulting products in aqueous acid media, led to enantioenriched beta-benzyl-gamma-hydroxymethyl-gamma-butyrolactones. The conformational equilibrium of enoates E- and Z-1b was evaluated by theoretical calculations (ab initio, MP2/6-31G), and a mechanistic rationale was proposed to explain the observed stereoselectivities.
authors.affiliation: Central Analítica, Núcleo de Pesquisas de Produtos Naturais, Centro de Ciências da Saúde, bloco H, Ilha da Cidade Universitária, Universidade Federal do Rio de Janeiro, Brazil, 21941-590.
authors.forename: André R G
authors.initials: AR
authors.lastname: Ferreira
authors.forename: Guilherme V M
authors.initials: GV
authors.lastname: de A  Vilela
authors.forename: Mauro B
authors.initials: MB
authors.lastname: Amorim
authors.forename: Kátia P
authors.initials: KP
authors.lastname: Perry
authors.forename: Antônio J R
authors.initials: AJ
authors.lastname: da Silva
authors.forename: Ayres G
authors.initials: AG
authors.lastname: Dias
authors.forename: Paulo R R
authors.initials: PR
authors.lastname: Costa