Assignment of the Absolute Configuration to Enantiomeric 2,3-Dihydro-1-phenyl-1H-phosphole 1-Sulfide. X-Ray Structure of (+)-(1S,3aR,7S,8S,8aR,8bR)-7,8-Di(tert-butoxy)-1-phenyloctahydro-1H-pyrrolo[1,2-b]-1H-phospholo[2,3-d]isoxazole1-sulfide

The structure and absolute configuration of the predominant of the two diastereomeric 7,8-di-tert-butoxy-1-phenyloctahydro-1H-pyrrolo[1,2-b]phospholo[2,3-d]isoxazole 1-sulfides obtained in the kinetic resolution experiment involving cycloaddition reaction of (S,S)-3,4-di-tert-butoxy-3,4-dihydro-2H-pyrrole 1-oxide (I) with an excess of racemic 2,3-dihydro-1-phenyl-1H-phosphole I-sulfide (2), was determined by a single-crystal X-ray diffraction technique. C22H34O3NPS, trigonal, space group P3(1), a = b = 11.831(1), c = 14.761(2) Angstrom, alpha = beta = 90 degrees, gamma = 120 degrees, V = 1789(4) Angstrom(3), Z = 3. The structure was solved by direct methods and was refined by full matrix least-squares calculations to R = 0.031 and R(w) = 0.036 using 3737 unique reflections with I > 3 sigma(I). The absolute configuration of the studied molecule was determined by calculation of the ETA (eta) parameter and was found to be 1S, 3aR, 7S, 8S, 8aR, 8bR. This finding assigns unequivocally the S configuration to the faster reacting enantiomer of 2 and correspondingly the R configuration to the isolated slower reacting (-)-2. It also confirms fully the notion that in the cycloaddition transition state 1 and 2 prefer to approach each other in the exo mode and from the ring faces opposite to P-Ph and C3-O-t-Bu substituents, respectively.