Reliable (13)C NMR Method of Making Relative Stereochemical Assignments to Certain N-[alpha-Hetero-beta-hydroxy(acetoxy)-beta-(substituted phenyl)-1'-oxopropyl]-2-oxazolidinones

A Triple Diene-Transmissive Diels−Alder Strategy To Build the Quassinoid Framework

[chemical reaction: see text]. An advanced intermediate to the highly oxygenated triterpene quassinoids was prepared in 14 steps from tetrahydrofuran. The key steps are three diene-transmissive Diels-Alder cycloadditions. Several features of this synthesis are noteworthy, including a successful Mitsunobu reaction on an allenylic alcohol, a rare [4 + 2] cycloaddition involving an enethiol ether dienophile, and complete control over all 10 chiral centers created.

An NMR method for assigning relative stereochemistry to beta-hydroxy ketones deriving from aldol reactions of methyl ketones

We describe a simple 1H NMR analysis that permits the stereochemistry of beta-hydroxy ketones to be assigned by visual inspection of the ABX patterns for the alpha-methylene unit of the beta-hydroxy ketone in the 1H NMR spectra. This method has been verified by application to a wide range of beta-hydroxy ketones deriving from aldol reactions of chiral aldehydes with a variety of chiral and achiral methyl ketone enolates (see Tables 1 and 2). The stereochemistry of 54 of these compounds have been assigned by rigorous chemical methods.