Stereocontrolled Synthesis of Functionalized Bicyclic α-Methylene Butyrolactones via Tungsten-Mediated Intramolecular Allylation of Aldehydes

Abstract

The syntheses of a series of CpW(CO) 2 (π-γ-lactonyl) complexes bearing a tethered aldehyde are described. These π-allyl complexes are prepared as either syn-or anti-stereoisomers. Treatment of these dicarbonyl complexes with NOBF 4 and NaI in CH 3 CN effects an intramolecular allylation of the tethered aldehyde, yielding bicyclic R-methylene butyrolactones comprising a homoallylic alcohol. Both syn-and anti-isomers of tungsten-π-allyl compounds produce the same R-methylene butyrolactones. The cyclizations proceed with high diastereoselectivities to give only the cis-fused bicyclic γ-lactones. The preference for cis-fused stereoselection can be rationalized based on a tricyclic transition state mechanism.

Synthesis of Heterocyclic and Carbocyclic Compounds via Alkynyl, Allyl, and Propargyl Organometallics of Cyclopentadienyl Iron, Molybdenum, and Tungsten Complexes

“…Tungsten−π-allyl complexes are elaborated for stereocontrolled synthesis of cis -fused α-methylene butyrolactones bearing a homoallylic alcohol . The synthetic protocol is shown in Scheme , which uses chloropropargyl substrates bearing an alcohol and an aldehyde.…”

“…This information suggests that the reaction mechanism involves a tricyclic transition state with aldehyde coordinating to tungsten center. An open transition state is expected to yield cis -fused bicyclic lactone with an anti -homoallylic alcohol . A semi-emperical calculation with the pm3(tm) using the program suit SPARTAN 5.0 suggests that the observed product has the lowest energy in its transition state among the four possible products.…”