ACYCLIC STEREOCONTROL VIA CYCLIC PRECURSORS

It is apparent from preceding sections that stereocontrol in cyclic systems is much easier than in acyclic systems, which is due, of course, to the conformational bias inherent in cyclic systems. Synthetic chemists have exploited this fact for many years. A cyclic system can be used to position functional groups, often with control of regio- and stereochemistry. The ring is then opened to give an acyclic system and the regiochemistry and stereochemistry of the substituents has been fixed. There are many examples. 

NaBH4 OH OH 2. Li2CuCl4, THF 2 equiv n-C7Hj5MgBr 90% n-C7Hi5 N 2. Br - C02H 90% -C7H15 

The diastereoselectivity obtained for reactions on a ring makes it possible to use a cyclic molecule to fix a stereocenter in an acyclic target, as seen in the formation of 154 from 152. Diol 152 had been prepared by a multistep sequence, with control of the stereocenters. When this diol was subjected to oxidation with MCPBA lactone 153 was formed. Subsequent ring opening with methanolic potassium carbonate led to the acyclic fragment 154, with six contiguous stereogenic centers whose stereochemistry had been fixed in the cyclic 

Macrocyclic ring compounds (sec. 6.6.B) possess functionality and stereochemistry that can be as difficult to control as in acyclic systems. The use of cyclic precursors is of value here also. In the Corey synthesis of (-j-A-methylmaytansine, a sugar derivative, tri-O-acetyl-D-glucal (155), was used as a precursor to 156. This precursor contained the correct stereochemistry and functionality for elaboration of the eastern 

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