Of dimethylhydrazones

Modem technology has influenced these reactions. Ultrasound assisted versions of these reactions has been reported. Ultrasound irradiation facilitated the Diels-Alder reaction of dimethylhydrazone 94 with 95. The resultant pyridine 96 are afforded in shorter reaction times and increased yields. 

Scheme 5 details the asymmetric synthesis of dimethylhydrazone 14. The synthesis of this fragment commences with an Evans asymmetric aldol condensation between the boron enolate derived from 21 and trans-2-pentenal (20). Syn aldol adduct 29 is obtained in diastereomerically pure form through a process which defines both the relative and absolute stereochemistry of the newly generated stereogenic centers at carbons 29 and 30 (92 % yield). After reductive removal of the chiral auxiliary, selective silylation of the primary alcohol furnishes 30 in 71 % overall yield. The method employed to achieve the reduction of the C-28 carbonyl is interesting and worthy of comment. The reaction between tri-n-butylbor-... 

Gawlcy and co-workers2 report that KDA is superior to LDA and to n-BuLi for deprotonation of dimethylhydrazones (7, 126-130) or of oxime ethers. The reaction is generally complete in THF at —78° in 15 minutes or less. The potassium counterion does not interfere with cuprate formation or conjugate addition. 

VandenHeuvel and Horning [379] dealt with the preparation of dimethylhydrazones of ketosteroids an oxo group in position 3, non-conjugated with a double bond, reacts with dimethylhydrazine alone. The preparation of hydrazones in positions 16, 17 and 20 and 3 conjugated with a double bond requires acid catalysis the 11-keto group does not react. SE-30 or QF-1 (1%) were satisfactory stationary phases. Condensation with other reagents and preparation of substituted hydrazones can serve for the characterization and identification of ketosteroids [380]. 

Arrieta et al. (1998) illustrated the synthesis of (4,3 )-bipyrazole (3) by utilizing the mixture of dimethylhydrazone and ethyl phenylpropiolate under microwave irradiation. They also described the microwave-assisted 1,3-dipolar cycloadditions with electron-dehcient dipolarophiles to afford the corresponding cycloadducts. The use of pyrazolyl hydrazones led to valuable compounds, such as bipyrazoles (4), in good yields, providing a new approach to the preparation of these heterocyclic derivatives. 

The direct sulfenylation of. / -dimethylhydrazones via the reaction of the a-lidiio derivative (14) with dimethyl disulfide (Scheme 13) has been reported, and the initially formed product (15) shown to isomerize to the more stable ( )-isomer (16). ) le further transformations have been carried out on compound (16), attempts do not ai rear to have been made to introduce unsaturation by the elimination of the thiol group. 

The Neber rearrangement of dimethylhydrazone quaternary salts to a-amino ketones is a well-studied reaction. However, when this reaction is carried out at temperatures below those of normal Neber condi-... 

Regiospecific alkylation of dimethylhydrazone anions with the masked acrolein equivalent, 3-bromo-propionaldehyde dimethyl acetal, has brcn used as an alternative to a conventional Michael reaction in Corey s total synthesis of picrotoxinin (c/. equation 13). Azaallyllithium reagents derived from aldehyde and ketone hydrazones, unlike enolates, yield monoalkylation products with control of both regio-chemistry and stereochemistry. In appropriate cases, alkylation followed by deprotection to form a dicarbonyl product can be a very effective synthetic strategy. 

Hydrolysis of dimethylhydrazones. Regeneration of carbonyl compounds from the... 

Hydrolysis of dimethylhydrazones. The regeneration of carbonyl compounds is promoted by Pd(OAc),-SnCL. 

For use of dimethylhydrazones in a conversion of aldehydes into nitriles see Lithium diethylamide-Hexamethylphosphoric triamide (this volume). 

A secondary amide is obtained by selective oxidation of a tertiary carbon center in adamantane with NaI04 in the presence of iron(III) perchlorate in acetonitrile (eq 18). Dimethylhydra-zones undergo periodate induced hydrolysis, at pH 7, to give carbonyl compounds in high yields (eq 19). However, these conditions are unsuitable for the hydrolysis of dimethylhydrazones derived from aromatic or a,unsaturated aldehydes because mixtures of aldehydes and nitriles are formed. 

Ultrasound irradiation facilitated the Diels-Alder reaction of dimethylhydrazone 252 with acetylene 253 to afford pyridine 254 in shorter reaction time and increased yields in comparison to the conventional heating conditions. ... 

Deprotonation of dimethylhydrazone derivatives of ketones takes place to give the anti-anion initially this anion rearranges to the 5y -anion by isomerization at nitrogen. Previous work had shown that these anions are readily alkylated. 

The oxidative final step can be neatly avoided by the use of hydroxy-lamine, " instead of ammonia, when a final 1,4-loss of water produces the aromatic heterocycle. In an extension of this concept, the construction of a 1,5-diketone equivalent by tandem Michael addition of dimethylhydrazone anion to... 

Metalation of dimethylhydrazone derivatives of aldehydes and ketones occurs cleanly. Unsymmetrical ketones suffer proton abstraction from the lesser-alkylated a-carbon atom specifically, and the a-lithiated dimethylhydrazones react more vigorously than the corresponding enolates with halides (Scheme 23), oxirans, and carbonyl compounds (Scheme 23). Cuprate derivatives can be obtained from the lithiated species in the usual manner and the cuprates undergo Michael addition to ajS-unsaturated ketones. 

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