Abstract
The C-24 configurations of (22E)-24-ethyl-24-methylcholesta-5,22-dien-3β-ol (1) and 24-ethyl-24-methylcholest-5-en-3β-ol (2), isolated from the Colombian Caribbean sponge Topsentia ophiraphidites, were determined to be R and S, respectively, by comparing their NMR data with those of stereodefined (24R)- and (24S)-samples that were synthesized in routes involving the orthoester Claisen rearrangement of Δ23-22-allylic alcohols. This is the first synthetic study where the Claisen rearrangement is used to introduce a C-24 quaternary center in a stereospecific manner with acceptable yield. X-ray analysis of 1 confirmed these stereochemical assignments.
Highlights
It is well known that marine organisms such as sponges and corals are rich sources of uniquely modified sterols in the side-chain.[1]
Sterol 1 was isolated from the marine sponge Psammocinia bulbosa and its C-24 configuration was inferred as R without any discussion on the stereochemistry.[5]
In a previous paper we reported the isolation of a number of multiply alkylated sterols, including compounds 1 and 2, from the Colombian Caribbean sponge, Topsentia ophiraphidites.[8]
Summary
It is well known that marine organisms such as sponges and corals are rich sources of uniquely modified sterols in the side-chain.[1]. In this paper we report a stereochemical determination at C-24 of 24-ethyl-24-methylsterols 1 and 2 (Figure 1), along with the 13C NMR data for the stereodefined C-24 epimers of 1 and 2. The 1H and 13C NMR spectroscopic data of the synthetic diastereomers 1a and 1b are listed in Tables 1 and 2, respectively, together with those of the sterol 1 isolated from T. ophiraphidites.
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