Unsaturated tosylhydrazones

With electron-deficient aromatic substrates (Entries 4 and 5), high yields and selectivities were observed, but enantioselectivities were variable and solvent-de-pendent (compare Entry 6 with 7 and see Section 1.2.1.3 for further discussion). With a,P-unsaturated tosylhydrazone salts, selectivities and yields were lower. The scope of this process has been extensively mapped out, enabling the optimum disconnection for epoxidation to be chosen [10]. 

When ot, 3-unsaturated aldehydes were employed, vinylepoxides were obtained with excellent transxis ratios but in poor yields. When benzaldehyde was treated with a, 3-unsaturated tosylhydrazone salts, the yields of vinylepoxides were improved but the transxis ratios dropped. When chiral sulfides were utilized, the ees were high with a, 3-unsaturated aldehydes, whereas unsaturated, chiral sulfur ylides gave moderate ees, poor yields, and modest transxis ratios. 

NaBH3CN, or bis(triphenylphosphine)copper(I) tetrahydroborate.531 The reduction of a, (3-unsaturated tosylhydrazones with NaBH3CN, with NaBH -HOAc, or with catecholborane proceeds with migration of the double bond to the position formerly occupied by the carbonyl carbon, even if this removes the double bond from conjugation with an aromatic ring,532 e.g.. 

Oxidation of the unsaturated tosylhydrazones 36 affords a mixture of ring-cleaved (38) and hydrolyzed (39) products (Scheme 6)42. The formation of 38 and 39 can be explained in terms of a common intermediate (37). Oxidation of acylhydrazides (40) follows an apparently mechanistically similar course to afford cyclized materials43 except in the presence of cyanide, in which case nitriles are formed44 (Scheme 7). 

Treatment of a,p-unsaturated tosylhydrazones with NaBH4 in MeOH affords principally allylic ethers from cyclic derivatives and pyrazoles with most noncyclic examples. This divergent behavior compared to saturated tosylhydrazones has been attributedto a lessening of the electrophilicity of conjugated imine ir-bonds, which allows initial abstraction of the acidic N—H proton by BH4 to compete with reduction, and gives alternative reactions related to the Bamford-Stevens process as depicted in Scheme 4. An exception to this may be the deoxygenation of conjugated vinyl triflates (entry 11, Table 6). The cyclopropanation and elimination products produced in entry 4, Table 6 also probably arise from similar, alternative reaction paths. ... 

In situ generation of diazoalkenes can be performed using alkali-metal salts of a,j -unsaturated tosylhydrazones. An early intramolecular application of this method was the photolysis of the sodium hydrazonide 22 to give the sesquiterpene thujopsen (23, 4%). However, the major product formed was the cyclopropene 24, generated via an intramolecular insertion reaction. ... 

Not all vinylcarbenes will cyclize to the cyclopropene. For example, those with an acyl substituent on the carbene-bearing center preferentially undergo a Wolff rearrangement, but the most significant limitations to the scope of this synthetic route arise from the problems of making the required 3//-pyrazoles. Many routes exist, but as far as cyclopropene synthesis is concerned, the 3/f-pyrazoles have been made (1) by addition of diazoalkanes to alkynes (2) from dihydropyrazoles (3) from cyclization of a,/l-unsaturated tosylhydrazones and finally (4) by modification of the structure of other 37/-pyrazoles. 

Unsaturated ketones, 211-212 a,S-Unsaturated 7-lactones, 200 0,7-Unsaturated methyl sulfinates, 381 a,0-Unsaturated nitriles, 351, 394, 535 0,7-Unsaturated nitriles, 84 a,0-Unsaturated sulfones, 42-43, 436 c ,(3-Unsatutated sulfoxides, 193, 394 a,0-Unsaturated tosylhydrazones, 537 ff, 7-Unsaturated-6 -valerolactones, 188 Uracil, 639, 640... 

The use of azoalkanes as synthetic intermediates is illustrated in Schemes 17 and 18. We also note the general methodof formation of azoalkanes via intramolecular cyclization of unsaturated tosylhydrazones under acidic conditions (Scheme 19). The reaction of (103) gave products of intermolecular condensation. Structure (104) was established by X-ray analysis. Use of the optically active triazolinedione (105), derived from camphor, has enabled the synthesis of optically active 4,5-diazatwist-4-ene (106), and the absolute... 

Reduction of a unsaturated tosylhydrazones. Hutchins et al. reported briefly that reduction of tosylhydrazones of aliphatic a,/8-unsaturaled ketones produces alkenes resulting from migration of the double bond to the carbon originally bearing the carbonyl group. Taylor and Djerassi have since examined this reduction in detail with steroidal substrates and found that the situation is more complex. Thus reduction of the tosylhydrazone of A -cholestenone gives the four products shown in equation (I). Reduction of the tosylhydrazone of A - -cholestadienone-3 is even more complex (equation II). 

Reductive deoxygenation of most a, -unsaturated tosylhydrazones with NaBHsCN cleanly affords alkenes in which the double bond migrates to the former tosylhydrazone carbon (eq 20). However, the process gives mixtures of alkenes and alkanes with cyclohexenones (see also Bis(triphenylphosphine)copper(I) Borohydride, Catecholborane and Sodium Borohydride). 

Synthesis.—Catecholborane offers a number of advantages over other boron hydrides for the reduction of a)3-unsaturated tosylhydrazones (1) to olefins. Only one equivalent of hydride is used and the conditions are mild. No alkane formation is observed. A study of the isomeric pulegone tosylhydrazones (2) and (3) and the... 

It has been pointed out °° that caution should be exercised in planning alkene syntheses through the reduction of a/3-unsaturated tosylhydrazones with sodium cyanoborohydride. Alkene (imminium ion reduction) and alkane (Michael addition to the unsaturated system) production can be competing processes. However, the new, mild procedure using catechol borane as the reducing agent seems to circumvent these difficulties. ... 

Borane-mediated reduction of a,p-unsaturated tosylhydrazones is another reliable reaction to synthesize alkenes. In this type of reaction, new C=C bonds are formed by an alkene walk within the substrates through intramolecular rearrangement [18-21]. 

Recently, Armesto, Horspool, and collaborators reported a novel photochemical cyclization of acyclic P,y-unsaturated tosylhydrazone leading to dihydropyrazoles. The acetophenone-sensitized irradiation of the 3,3-dimethyl-5.5-diphenylpent-4-en-2-one tosylhydrazone yielded the dihydropyrazole in 75% yield. The reaction path, analogous to the oxime cyclization mentioned above, involves SET from the alkene moiety to the substituent on the terminal nitrogen as outlined in Scheme 30. ... 

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