El-Awa Ahmad, du Jourdin Xavier Mollat, Fuchs Philip L
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
J Am Chem Soc. 2007 Jul 25;129(29):9086-93. doi: 10.1021/ja071217x. Epub 2007 Jun 14.
Enantiopure cycloheptadienyl sulfones 6 and 7 are diastereoselectively epoxidized to yield epoxyvinyl sulfones 8, 9, 14, and 16 in high yields and diastereomeric ratios. Syn and anti methylation of epoxides 8, 9, 14, and 16 enables access to all eight possible diastereomeric stereotetrads, seven of which are commonly found in polypropionate natural products. Anti methylations of the above epoxides are possible by either the reaction of methyl organometallics promoted by copper(I), or via reaction with trimethylaluminum to yield stereotetrads 11, 12, 22, and 24. Syn methylations are achieved via Lawton SN2' reaction in the case of stereotetrads 10, 15, and 38, while stereotetrad 13 is accessed by an oxidation/reduction alcohol inversion sequence from stereotetrad 11. All stereotetrads were obtained in high diastereomeric ratios and yields, and their relative stereochemistry was confirmed by X-ray crystallography. Oxidative cleavage of the cyclic stereotetrads yields termini-differentiated acyclic heptanyl stereotetrads ready for use in building larger fragments in the course of target syntheses.
对映体纯的环庚二烯基砜6和7被非对映选择性环氧化，以高收率和非对映体比例生成环氧乙烯基砜8、9、14和16。环氧化物8、9、14和16的顺式和反式甲基化能够得到所有八种可能的非对映体立体四元体，其中七种常见于聚丙酸酯天然产物中。上述环氧化物的反式甲基化可通过铜（I）促进的甲基有机金属化合物反应，或通过与三甲基铝反应生成立体四元体11、12、22和24来实现。对于立体四元体10、15和38，顺式甲基化通过劳顿SN2'反应实现，而立体四元体13通过从立体四元体11的氧化/还原醇转化序列得到。所有立体四元体均以高非对映体比例和收率获得，其相对立体化学通过X射线晶体学得到证实。环状立体四元体的氧化裂解产生末端差异化的无环庚基立体四元体，可用于在目标合成过程中构建更大的片段。
