Enantiocontrolled [5+2] cycloaddition to η3-pyranylmolybdenum π-complexes.: Synthesis of substituted oxabicyclo[3.2.1]octenes of high enantiopurity
Enantiocontrolled [5+2] cycloaddition to η3-pyranylmolybdenum π-complexes.: Synthesis of substituted oxabicyclo[3.2.1]octenes of high enantiopurity
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DOI：
10.1021/ja990729b
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发表时间：
1999-06-23
影响因子：
15
通讯作者：
Liebeskind, LS
中科院分区：
文献类型：
--
作者：
Yin, JJ;Liebeskind, LS
Consider the synthetic potential of enantiomerically pure, stoichiometric transition metal π-complexes derived from an inexpensive metal source and from readily available chiral-pool organic substrates. Assume, hypothetically, that either enantiomer of such metal π-complexes could be prepared in quantity by simple synthetic procedures and that these transition metal π-complexes were sufficiently air-and moisture-stable to allow routine handling. Furthermore, assume that a single metal and its spectator ligands could facilitate and direct multiple and sequential regioand stereocontrolled transformations at the π-ligand (CC, CO, and CN bond formation, cycloaddition, oxidation, reduction) prior to a deliberate demetalation that would deliver an enantiomerically pure organic molecule. These metal π-complexes would then represent synthetically potent enantiomerically pure scaffolds for the asymmetric construction of a large variety of organic molecules.Following these principles, an enantiocontrolled synthesis of oxabicyclo [3.2. 1] octanes was conceived (Scheme 1). If 3-substituted pyranyl complexes η3-bound to a TpMo (CO) 2 moiety (Tp) hydridotrispyrazolylborato) could be prepared, a stepwise cycloaddition with electron-deficient alkenes would be anticipated (1 to 2 through intermediate 4). After a deliberate demetalation/functionalization sequence, substituted oxabicyclo [3.2. 1] octanes would be obtained. If the η3-pyranylmolybdenum complexes could be prepared in high enantiopurity, then high enantiopurity oxabicyclo-[3.2. 1] octanes would result. Higher-order cycloadditions to metal complexes are known. 1-4 Harman and Liu have described cycloadditions to η2-pyrrole5 and η1-3-furyl complexes, 6 respectively, and reactions initiated at the noncoordinated double bond of η3-pentadienyl and η3-cyclohexadienyl π-complexes are known. 7-9 The overall concept was validated through a study of the racemic η3-3-methylpyranylmolybdenum complex (()-6 and the corresponding (+)-6 antipode (97% ee). The former was easily prepared in 64% overall yield on a 12 g scale from 6-acetoxydihydropyran-3-one by way of the 3-oxopyranyl complex (()-5; the latter was generated on a 5 g scale from 6-hydroxy-2H-pyran-3 (6H)-one, which was resolved via the corresponding diastereomeric (R)-pantolactone-substituted-2H-pyran-3 (6H)-one (Scheme 2). 10 For the preparation of the enantiomerically enriched sample, the separate diastereomeric pyranones were converted into the corresponding π-allylmolybdenum complexes (-)-5 (61% yield, 95% ee) and (+)-5 (54% yield, 95% ee) following a procedure
