Carbamates epoxidation directed

In an additional variation on the theme of spirocyclization via oxidative rearrangement chemistry, oxidation across the indole 2,3-Jt system has been employed by Martin and coworkers in their approach to the core spirocyclic ring structure of citrinadin A [111]. The key epoxidation/spirocyclization was carried out in diastereoselective fashion using an indole A-chiral auxiliary, (—)-8-phenyl-methol carbamate, to direct the asymmetric event. 

Z)-awh-4-Hydroxy-l-aIkenyl carbamates 363, when subjected to substrate-directed, vanadyl-catalysed epoxidation , lead to diastereomerically pure epoxides of type 364 (equation 99)247,252,269 qqjggg epoxides are highly reactive in the presence of Lewis or Brpnsted acids to form -hydroxylactol ethers 366 in some cases the intermediate lactol carbamates 365 could be isolated . However, most epoxides 364 survive purification by silica gel chromatography . The asymmetric homoaldol reaction, coupled with directed epoxidation, and solvolysis rapidly leads to high stereochemical complexity. Some examples are collected in equation 99. The furanosides 368 and 370, readily available from (/f)-0-benzyl lactaldehyde via the corresponding enol carbamates 367 and 369, respectively, have been employed in a short synthesis of the key intermediates of the Kinoshita rifamycin S synthesis . 1,5-Dienyl carbamates such as 371, obtained from 2-substituted enals, provide a facile access to branched carbohydrate analogues . 

Peracids m-CPBA and CF3CO3H have been used in epoxidations of substrates with two mnable allylic directing groups expected to direct the peracids to opposite faces of the alkene. Control of face selectivity was observed and attributed to the different binding abilities of the two peracids to the various allylic functionalities, carbamate on one side of the alkene and alcohol, methyl ester, acetate, trifluoroacetate, or TBS-ether on the other. The conversion of A-mono-protected and A-di-protected cyclopent-... 

For example, the 5-(tert-butyldimethylsilyloxy)pentyl carbamate 159a was converted to the enantiomerically emiched (>95% ee) stannane 159b via sparteine-mediated deprotonation. Then an aUyl chloride unit was elaborated, finally, the (S)-lithium intermediate 179 was generated by lithiodestannylation. The (li ,2S)-2-vinylcyclopentyl carbamate 180 was produced with essentially complete enantio- and diastereoselectivity [Eq. (46)] [98]. AUyl chloride (178,H for Bu3Sn) and epoxides do not survive direct lithiation [117]. 

Epoxidation of Acyclic Alkenes. Stereoselective epoxidation of a series of allylic carbamate methyl esters (eq 34), homoallylic alcohols (eq 35), and acetates (eq 36) could be performed with good to excellent stereocontrol. It is believed that the directing effect of the carbamate protecting group plays an important role in dictating the level of stereocontrol. 

Although compounds of type 1 independent of the directing group give rise preferentially to the syn-epoxides (see 2), a simple shift of the double bond into the 1,2-position leads to diminished stereoselectivity, and if the N-benzyl carbamate is used as the directing group, a mixture of anti-epoxide and carbonate 6 is obtained, which can be focused into 6 on subsequent acid treatment. This outcome proves that in this case the anti-epoxide is by far the major product. 

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