Stereochemistry of Nucleophilic Addition Reactions. 10. Preparation of 2-C-Diacetylmethyl and 2-C-Acetylmethyl Derivatives with the Altro, Gluco, and Manno Configuration from Methyl 4,6-O-Benzylidene-2,3-dideoxy-3-nitro-α-d-erythro-hex-2-enopyranoside