Syn hydr~ boration~ of 4-2 leads to the erythro y-chloroborane (5). Similarly 4-E gives the threo diastereomer. Presumably cyclization involves coordination of the base with the boron atom to form an ate complex which undergoes 1, 3-eliminati~ n.~-~ The present results show that this proceeds with inversion of configuration of the carbon-boron center.
