Chain Extensions at the Primary Carbon Atom

A single diastereoisomer was also obtained by Ireland in the same context but using a chiral lithium derivative obtained from 216 by metal-halogen exchange [155]. However, the presence of magnesium bromide is needed to achieve up to 20 1 diastereoselectivity. 

SCHEME 11.49 Reagents i trimethylcrotylsilane then NaH, Mel ii (1) HCl, MeOH and (2) BF3-Et20, EtaSiH, CH2CI2 iii (1) (CH3)2C(OAc)COBr, CH2CI2, (2) Amberlite IRA 400(OH ), MeOH and (3) LiBHEtj iv (1) O3 then Zn-AcOH, (2) Ph3P=CHC02Me then DIBAL and iii. TBSCl v PCC. 

SCHEME 11.SI Reagents i Me2CuLi, MeLi, Et20 ii (1) CSA, MeOH, (2) SO3, pyridine, DMSO and (3) MeMgBr, THF iii (1) MeS02Cl, pyridine and (2) BU4NI, benzene iv Zn, EtOH. 

As alternatives to nucleophilic additions to C6 or C5 aldehydes, olefinations of the same compounds offer a repertoire of solutions to chain extensions or to the coupling of two carbohydrate building blocks. As shown in the above section, chain extensions at C6 are often followed by the introduction of new functional groups and thus new chiral centers. In this regard, olefins obtained via Wittig-type reactions are excellent starting compounds. 

241 as the major compound. Dihydroxylation of the double bond of 241 proceeded with a good selectivity (7 1) in favor of diol 242. 

---