Acylhydrazones acylhydrazines

Substituted acylhydrazones (95), formally derivatives of 1,2-diacylhydrazines (75), also cyclize to oxadiazoles (76), with loss of HX. Imidol ethers (95a R2 = H), prepared from acylhydrazines R1CONHNH2 and ortho esters HC(OR)3, cyclize on heating to monosub-stituted oxadiazoles (76 R1 = H). This reaction is an important synthesis of monosubstituted... 

The scope of the reaction was evaluated by variations in both the radical and the radical acceptor. In the presence of ZnCb, the propionaldehyde N acylhydrazone 8a was subjected to radical additions of various organic iodides (Table 2.3, entries 1 4). Reaction conditions entailed addition of Bu3SnH (5 equiv) and O2 (7 ml/mmol) by syringe pump to a mixture of iPrI (10 equiv), 136 (10 equiv), and Lewis acid (2 equiv) in 2 1 CH2Cl2/ether at 78 °C, gradually bringing the mixture to ambient temper ature after the addition. Under these conditions, ethyl radical (from the triethylbor ane) can compete for the radical acceptor, and as a result, the separable ethyl radical adduct 17 (Scheme 2.2) was observed (<10% yield) in all cases. With simple secondary and tertiary alkyl iodides as radical precursors (entries 1 4), additions to 8a occurred with moderate yields to afford N acylhydrazines. Radical reactivity is... 

Protection and Deprotection.—In a substantial paper, Barton and co-workers have discussed in detail the protection of carboxylic acid functions in penicillin derivatives by conversion into the corresponding acylhydrazines, acylhydrazones, and dihydro-heteroaromatic amides. An alternative method for the methoxymethyl-ation of free acids, which avoids the use of toxic chloromethyl methyl ether, is by prior conversion into the zinc bromide salt followed by reaction with acetyl chloride and methylaP (Scheme 14). 

Example 3.3 Af-acylhydrazones are more stable than imines, and their enantiose-lective reduction to Af-acylhydrazines is successfully completed by the catalytic Rh(I) complex of chiral bidentate phosphine ligand 22 (Scheme 3.5) [10]. 

Acylhydrazines from acylhydrazones, also with hydrogenolysis... 

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