Conversely, when A-alkyl tryptophan methyl esters were condensed with aldehydes, the trans diastereomers were observed as the major products." X-ray-crystal structures of 1,2,3-trisubstituted tetrahydro-P-carbolines revealed that the Cl substituent preferentially adopted a pseudo-axial position, forcing the C3 substituent into a pseudo-equatorial orientation to give the kinetically and thermodynamically preferred trans isomer." As the steric size of the Cl and N2 substituents increased, the selectivity for the trans isomer became greater. A-alkyl-L-tryptophan methyl ester 42 was condensed with various aliphatic aldehydes in the presence of trifluoroacetic acid to give predominantly the trans isomers.  [c.474]

Oxidations. Avermectin B and ivermectin have two secondary hydroxy groups susceptible to oxidation to the corresponding ketones. Because of the aHyhc nature of the C-5 hydroxy group its selective oxidation to 5-oxoavermectin is readily accompHshed with manganese dioxide (40). Although tautomerization to the enol and elimination of the remaining tertiary C-7 hydroxy group leads to a phenol (35), the 5-oxo derivatives are nonetheless stable enough to allow further chemical reactions. Particularly important is the stereospecific reduction with NaBH to the 5-beta alcohol with natural configuration, which is used to prepare tritiated avermectin derivatives. Reaction with trifluoroacetic anhydride, however, gives the 2,5,6,7-tetradehydro phenol analogue. Use of mild oxidation procedures, such as oxalyl chloride-DMF (Swem oxidation), gives 4 /5-dioxo avermectin derivatives or, after protection of the C-5 hydroxy group as a ferZ-hutyl dimethyl silyl derivative, 5-0-/ -butyldimethylsilyl-4"-oxoavermectins, which are valuable intermediates for further 4 -modifted analogues. Sodium borohydride reduction of the 4 -keto function gives a mixture of 4 -epimeric hydroxy compounds, with the unnatural axial 4 -hydroxy group the major product and the natural epimer the minor one. Reductive amination with NaCNBH and NH OOCCH likewise yields an epimeric mixture of 4 -aminoderivatives with interesting biological properties (41).  [c.283]

Those epimers of 1,2,3,4-tetrahydro-j8-carbolines of general structure 139, in which the hydrogen on carbon-1 of the carboline moiety is axial, are converted into the corresponding 3,4-dihydro- -carbolinium salts (141) by mercuric acetate oxidation. Sodium dichromate  [c.115]

It was recently shown that the formation of a Schiff s base is the first step of the reaction between 3-aminopropan-l-ol and an aldehyde. The action of acid chlorides, such as tosyl chloride, on the SchifTs base forms the W-acyl derivative of tetrahydro-1,3-oxazine.  [c.315]

Drefahl and Horhold discussed the mechanism of N 0 acyl migration in A -benzoyl-l,2-diphenyl-3-aminopropanols. The migration does not seem to be possible in the erythro isomer as it would give an intermediate tetrahydro-1,3-oxazine with a bulky phenyl group in axial position. Consequently the erythro isomer is cyclized with inversion to form 2,5,6-triphenyl-5,6-dihydro-l,3-4 -oxazine  [c.339]

See pages that mention the term C ARB AZOLE, 1,2,3,4-TETRAHYDRO : [c.281]    [c.281]    [c.704]    [c.746]    [c.492]    [c.105]    [c.339]    [c.80]    [c.50]    [c.278]    [c.306]   
Organic syntheses Biclormethyl ether (1956) -- [ c.30 , c.90 ]