Ethers, allyl thermolysis

The mechanism of thermolysis and photolysis of ethers of 3-hydroxy-1,2-benzisoxazole has also been studied. Heating of the allyl ether (43) gave minor amounts of (44) and two benzoxazoles. Photolysis of (45) in methanol gave a benzisoxazole and an iminoester, via intermediate (46). Thermolysis at 600 °C gave a benzoxazole, a benzoxazolone and cyano-phenol (Scheme 16) (71DIS(D)4483). 

Two approaches for the synthesis of allyl(alkyl)- and allyl(aryl)tin halides are thermolysis of halo(alkyl)tin ethers derived from tertiary homoallylic alcohols, and transmetalation of other allylstannanes. For example, dibutyl(-2-propenyl)tin chloride has been prepared by healing dibutyl(di-2-propenyl)stannane with dibutyltin dichloride42, and by thermolysis of mixtures of 2,3-dimethyl-5-hexen-3-ol or 2-methyl-4-penten-2-ol and tetrabutyl-l,3-dichlorodistannox-ane39. Alternatively dibutyltin dichloride and (dibutyl)(dimethoxy)tin were mixed to provide (dibutyl)(methoxy)tin chloride which was heated with 2,2,3-trimethyl-5-hexen-3-ol40. 

Using the above procedures, allyl a-azido alkyl ethers of type 281 were prepared by employing an unsaturated alcohol such as allyl alcohol [76] (Scheme 32). The reaction of an aldehyde with allyl alcohol and HN3 in a ratio of 1 3 9 carried out in the presence of TiCl4 as catalyst provided azido ethers 281, 283, and 285 in 70-90% yield. The ratio of reagents is critical to ensure a high yield of azido ether and to prevent formation of acetal and diazide side products [75]. Thermolysis of azido alkenes 281, 283, and 285 in benzene (the solvent of choice) for 6-20 h led to 2,5-dihydrooxazoles 282,284, and 286, respectively, in 66-90% yield. 

In view of the proposed mechanism for this cleavage reaction, it was reasonable to expect that certain allylic ethers for which an elimination pathway existed (Scheme II) would also be susceptible to cleavage under thermolysis, acidolysis or a combination of both. 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

An irreversible consecutive reaction as a driving force to shift an unfavorable Cope rearrangement equilibria in the needed direction can be illustrated by the Cope-Claisen tandem process used for the synthesis of chiral natural compounds243. It was found that thermolysis of fraws-isomeric allyl ethers 484 or 485 at 255 °C leads to an equilibrium mixture of the two isomers in a 55 45 ratio without conversion into any other products (equation 184). Under the same conditions the isomer 487 rearranges to give the Cope-Claisen aldehyde 491 (equation 185). Presumably, the interconversion 484 485 proceeds via intermediate 486 whose structure is not favorable for Claisen rearrangement. In contrast, one of the two cyclodiene intermediates of process 487 488 (viz. 490 rather than 489) has a conformation appropriate for irreversible Claisen rearrangement243. 

On thermolysis, appropriately substituted A-allyl-A-silyloxy enamines 19 undergo smooth [3,3]-sigmatropic rearrangements to the corresponding A-silyloxy imino ethers laP (equation 5). Two stereogenic centers are created but no reference to chiral induction is referred. High diastereoselectivity was observed and short reaction times favoured the syn A-silyloxy imino ether diastereomers. 

While many researchers have used the 1,3-APT process to generate cyclic nitrones, it is clear that the operating reaction pathway in the oxime to isoxazolidine conversion may not always be predicted. In the work of Aurich and co-workers (317,318), the polycyclic isoxazohdine 292 was isolated as the major product from thermolysis of oxime 293 and may have been formed via two separate reaction paths (Scheme 1.61). In the proven route, initial 1,3-APT of 293 formed a 1,4-oxazine nitrone (294), which acted as the acceptor for the second 1,3-APT with the remaining oxime function. The cyclic nitrone 295 so formed underwent a 1,3-dipolar cycloaddition with the allyl ether forming the isolated polycyclic isoxazolidine adduct 292. 

In another study several simple silenes RR Si=CH2 (R, R = Me, Vinyl etc.) were formed by laser-powered pyrolysis and were found to form linear polymers, in contrast to the usual behavior of silenes which yield cyclodimers when formed by conventional thermolysis techniques16. Reactions of the silenes in the presence of several monomers such as vinyl acetate, allyl methyl ether and methyl acrylate were also studied. Laser-induced decomposition of silacyclobutane and 1,3-disilacyclobutane gave rise to silenes and other oxygen-sensitive deposits17,18. 

Ring opening of the oxaspiropentane 343 upon treatment with sodium phenylselenide (vide supra, Sect. 4.5, Eq. (34)) 59) and O-silylation produce the vinylcyclopropanol trimethylsilyl ether 344 which, on flash thermolysis at 670 °C, gave the siloxycyclo-pentene 345 as a 2 1 mixture of epimers at C(8). Then, allylation of the more substituted enolate arising from 345, opens a convenient way to the antitumor agent, aphidicolin 346 181>. 

Chelotropic addition of dichlorocarbene to bornadiene gave (200 X = C1) whereas with difluorocarbene the, yyn-adduct was favoured over the anh-adduct (200 X = F) (cf. Vol. 3, p. 59). A thio-Claisen rearrangement has been reported with the allylic enethiolic ether of thiocamphor. Flash thermolysis of allyl exo-2-bornyl sulphide to thiocamphor and propene has been examined. 

The Claisen rearrangement can be effectively catalyzed by Lewis acids, Bronsted acids, bases, Rh(I) and Pt(0) complexes as well as by silica . Several reviews were published recently in which the application of zeolites and acid-treated clays as catalysts for the Claisen rearrangement was described Thus, it was shown that the rearrangement conditions for phenolic allyl ethers can be dramatically milder if this reaction is carried out by thermolysis of a substrate immobilized on the surface of previously annealed silica gel for chromatography. For example, the thermolysis of ether 159 on silica gel (in a 159 Si02 ratio of 1 10 w/w) at 70°C gives the phenol 160 in 95% yield after 3.5 hours (equation 70). An additional example is shown in equation 71. ... 

The required chain extension of 12 was accomplished via deprotonation with NaH and condensation with aldehyde 7 to afford the Diels-Alder precursor 13 in 50% yield. Thermolysis of triene 13 and lactam 3 in xylene at 170 C for four days resulted in the desired cycloaddition to 14. Chromatographic purification permitted isolation of pure 14 in addition to a small amount of an exo isomer (>4 1 ratio). Acid treatment induced cleavage of both the silyl ether and acetonide. Reprotection of the diol and selective epoxidation of the A olefin produced 15 in 64% yield from 12. Epoxide 12 was then transformed to the isomeric allylic alcohol 16 by conversion of the alcohol to the bromide followed by reductive elimination. Protecting-group manipulation and subsequent oxidation the gave aldehyde 17, which was homologated and hydrolyzed to give seco acid 18 in 32% overall yield from 16. 

The nucleophilicity of sulfur and its ability to stabilize a-carbanions provide sulfur compounds with unique opportunities for sigmatropic processes consecutive rearrangements are no exception. The formation of salt (140) via Sn2 alkylation of ( )-2-butenyl bromide (139) followed by deprotonation leads to the intermediate allyl vinyl ether (141) which, under the conditions of the deprotonation, undergoes a thio-Claisen rearrangement to afford thioamide (143 Scheme 10). Thermolysis of (143) at elevated temperature affords the Cope product (142) in addition to some of its deconjugated isomer. Several unique characteristics of the thio-Claisen sequence should be noted first, the heteroatom-allyl bond is made in the alkylation step, this connection teing not notrtudly practised in the parent Claisen reaction ... 

Several routes to 4-alkyl-2,5-dihydrooxazoles, which were not available by previous methods, have been discovered. Thermolysis of allyl a-azidoalkyl ethers (181) gives 2,5-dihydrooxazoles via triazoline intermediates <88JOC27>. The starting materials are obtained from aldehydes, allyl alcohol, and hydrazoic acid (Scheme 89). A hlorination of oxazolidines with r-butyl hypochlorite, followed by dehydrochlorination using potassium superoxide, also provides 4-alkyl-2,5-dihydrooxazoles in 40-93% yield <92TL7751>. 

Allyl vinyl ethers have been prepared by acid-catalyzed exposure of an aldehyde to an allylic alcohol followed by thermolysis of the bis-allyl acetal150. 

The equatorial/axial selectivity in the 1,1 rearrangement of the arylsulfonylmethyl-substitut-ed allyl vinyl ether 9 depends on the reaction conditions. Whereas thermolysis of 9 results in a 2 1 mixture of 10 and 11 comparable to the thermal rearrangement of allyl vinyl ether 12 " l7, the anionic rearrangement of 9 gives a 2 1 mixture in favor of the product 11104. 

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