Postulated intramolecular sulfur

Figure 6. Postulated intramolecular sulfur incorporation into A7,22- and A7,24(28)-sterols. Reactions in an intermolecular fashion do not seem to...

The molecule 42 is located on an inversion center and appears to be essentially planar, the dihedral angle between the two independent thiophene rings amounting to 2.5(5)°. One interesting feature is the short intramolecular S(l)-S(2) distance (3.066(5) A) when compared with the Van der Waals distance (S-S 3.7 A), indicating a degree of delocalization between the two sulfur atoms, as already postulated in various molecules with the same 1,5-sulfur-sulfur interaction where S-S distances around 3 A were observed <1996JOC6997>. 

In 2008, the group of Williams [15] reported the first total synthesis of ( )-2-0-methylneovibsanin H (33) (Scheme 14.6). The diterpene, isolated from the leaves of Viburnum awabuki [16], was prepared by employing an acid-catalyzed domino sequence. Thus, the key transformation was the conversion of advanced intermediate 28 to cyclohexene 32 upon treatment with an excess of sulfuric acid in anhydrous methanol. Acid-mediated silyl deprotection first revealed alcohol 29, which readily underwent an intramolecular oxa-Michael addition to yield tricyclic 30. It was postulated that solvolysis and nucleophilic addition of methanol to the intermediary allyl cation then furnished acid 31, which underwent Fischer esterification. The resultant highly functionalized cyclohexene 32 was isolated in 50% yield as a mixture of diastereomers at C2 (diastereomeric ratio (dr) = 85 15). The observed stereochemistry at the newly created stereocenters, i.e., at C2 and C5, was postulated to arise from the preexisting sterically congested stereocenters in the starting material (i.e., 28). Cyclohexene 32 was eventually taken on to provide the natural... 

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