4-Pentenols, oxidation

The greater activity of Ti-beta (vs. TS-1) in the oxidation of the bulky cyclohexane was noted in the previous section. Table IX provides a comparison of the conversion and epoxide selectivity in the reaction catalyzed by TS-1 and three large-pore/mesoporous Ti-silicates in the epoxidation of a single, linear allyl alcohol (pentenol). 

We have studied the anodic oxidation of unsaturated alcohols using the controlled potential electrolysis (E = 1.9V vs SCE) in CH3CN-O.I mol/1 Et4NC104 solution in a divided cell [110]. The oxidation of 4-pentenol after consumption of 0.8 F/mol gave 2-methyltetrahydrofuran and tetrahydropyran as the major products. The oxidation of 5-pentenol gave 2-methyltetrahydro-pyran and oxepam, while the oxidation of 3-butenol under the same reaction conditions did not give the cyclic products. We rationalized this reaction as the electrongenerated acid (EGA) catalyzed intramolecular cyclization (Scheme 44). 

The series of reactions leading to the 5-silyl-l-pentene - epoxidation, ring expansion, and Peterson elimination -are all stereospecific. Therefore, epoxides with different geometry can be transformed into the corresponding (E)- or (Z)-olefinic silanols <1994BCJ1694, 1991TL4545>. Subsequent Tamao oxidation affords stereodefined pentenols. 

Keto aldehydes are prepared by the allylation of aldehydes. Reaction of 2-p-tolylpropanal (27) with allyl bromide gave 2-p-tolyl-2-methyl-4-pentenol (28) in 63% yield. The oxidation of the terminal... 

The stereoselective preparation of rfl .v-2-hydroxymethyl-5-subslituted tetrahydrofurans from 4-pentenols is possible by oxidative cyclization catalyzed by cobalt(ll) salts. Both cobalt(II) trifluoroacetate and cobalt acetylacetonate have been employed for the cyclization, but better yields are obtained with Co(modp)2 (l)45. 

Pentenols. Alkenes undergo allylboration with allyldibromoborane. Oxidation of the adducts gives 2-substituted 4-pentenols. 

A soln. of diphenyl disulfide in 0.1 M tetra- -butylammonium bromide (as both mediator and electrolyte) in dichloromethane electrolysed under argon in a divided cell with platinum electrodes at a constant current of 0.02-0.03 A (current density 3 mA/cmO until 10 As of charge consumed, 0.5 eqs. 4-pentenol added, and electrolysis continued until reaction complete by g.c. or t.l.c. 2-(phenylthiomethyl)tetr hydro-furan. Y 72%. The method is inexpensive, less toxic than Se-based methodologies, and generally applicable to 5- and 6-membered O-heterocyclics reaction may also be achieved (preferably) by direct electrolysis with bis(4-methoxyphenyl) disulfide (having a much lower oxidation potential than diphenyl disulfide). F.e. and stereoselectivity, also 2-(aryIthio)lactones from ethylenecarboxylic adds, s. S. Toteberg-Kaulen, E. Steckhan, Tetrahedron 44, 4389-97 (1988). 

Oxidative cyclization using Co(nmp)2 is a cost-effective and scalable route to form a trans-THF ring from pentenols, leading to amphidinolide C (13T8632). 

As an important milestone in this development one may regard the Mukayama oxidative cyclization of pentenols (see 330) [109]. 

Vanadium was found to be active in the direct transformation of 4-pentenols to 3-acyloxy-y-butyrolactones, using TBHP as oxidant... 

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