S-shaped diastereomers

The selective heterodimerization of glycoluril NH-compounds and glycoluril cyclic ethers (Section 4.4.1.1) and the general preference for the C-shaped diastereomers suggested that the mechanism of S-shaped to C-shaped equilibrium might proceed by an intramolecular rather than an intermolecular process under anhydrous acidic... 

SCHEME 15. Three different mechanisms for the equilibrium between the S- and C-shaped diastereomers. 

Simmons-Smith cyclopropanation, 456 Simmons-Smith reaction, 455 six-centered transition state, 264 Smiles-type rearrangement, 7 SOCI2, 19, 159 sodium azide, 365, 375 sodium borohydride, 416 sodium hexamethyldisilazide, 290 sodium hydride, 372, 375. 383 sodium in liquid ammonia, 440 sodium iodide, 379 sodium phenylselenide, 458 sodium trifluoroethoxide, 447 solid phase synthesis, 25 Sonogashira conditions, 409 Sonogashira coupling, 411 spartadienedione, 144 spontaneous csdodimerization, 23 S-shaped and C-shaped diastereomers, 83 stainless steel reactor, 283... 

Although unexpected, the above reduction and methanol addition process does appear to be well established. For instance, the relevant product, 5.96, has been subject to X-ray diffraction analysis (Figure 5.5.5). The structure so obtained revealed that complex 5,96 is bowl shaped, and does, indeed, possess a meso-position that has been converted into a methoxy-bearing sp -hybridized center. Consistent with this structure and the addition of methanol it implies, is the observed loss of a Soret-like band in the visible spectrum of 5.96. Also consistent with it is the complex nature of the H NMR spectrum. Complex 5.96 actually exists as four stereoisomers (an enantiomeric set of diastereomers). These stereoisomers were found to interconvert readily in solution. This interconversion is slow on the NMR time scale. Thus, all species are seen in the NMR spectrum and this, as well as the loss in s)mimetry, contributes to the complexity of the H NMR spectrum. 

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