Rearrangement protocols reactions

The acetal [1,2]-Wittig rearrangement protocol is also applicable to the synthesis of medium-sized cyclic ethers. For example, a reaction of the 9-membered cyclic acetal 37 with lithium piperidide provides the 8-membered ring ether 38 in good yield along with high diastereoselectivity (equation 20) . 

Pinacol Rearrangement in Tandem with the Carbonyl-Ene Reaction. Overman and co-workers have developed protocols in which pinacol rearrangement... 

In 1993, a first application of the Walters protocol in natural product syntheses was reported [54] AT-allylamide 262 could be converted into a 1 1 mixture of the diastereomer nitriles 263 with 56% yield. Despite the mild reaction conditions, no external 1,2-asymmetric induction (remote stereo control) was operative when conducting such a rearrangement. The diastereomers were sep-... 

When the commodity chemical propylene oxide is heated to high temperature in the gas phase in a shock tube, unimolecular rearrangement reactions occur that generate the CsHgO isomers allyl alcohol, methyl vinyl edier, propanal, and acetone (Figure 15.9). Dubnikova and Lifshitz carried out a series of calculations to determine the mechanistic pathway(s) for each isomerization, with comparison of activation parameters to those determined from Arrhenius fits to experimental rate data to validate the theoretical protocol. 

The allyl-transfer reaction based on 2-oxonia Cope rearrangement allows highly stereocontrolled chirality transfer. Triflic acid has been shown to induce the rearrangement of the 251 allyl sterols into 22-homoallylic sterols with high stereoselectivity without side reactions86 [Eq. (5.316)]. The protocol, however, is not effective for syn substrates (for example, 251, R = H, R = COOEt). 

The cyclohexene 121, which was readily accessible from the Diels-Alder reaction of methyl hexa-3,5-dienoate and 3,4-methylenedioxy-(3-nitrostyrene (108), served as the starting point for another formal total synthesis of ( )-lycorine (1) (Scheme 11) (113). In the event dissolving metal reduction of 121 with zinc followed by reduction of the intermediate cyclic hydroxamic acid with lithium diethoxyaluminum hydride provided the secondary amine 122. Transformation of 122 to the tetracyclic lactam 123 was achieved by sequential treatment with ethyl chloroformate and Bischler-Napieralski cyclization of the resulting carbamate with phosphorus oxychloride. Since attempts to effect cleanly the direct allylic oxidation of 123 to provide an intermediate suitable for subsequent elaboration to ( )-lycorine (1) were unsuccessful, a stepwise protocol was devised. Namely, addition of phenylselenyl bromide to 123 in acetic acid followed by hydrolysis of the intermediate acetates gave a mixture of two hydroxy se-lenides. Oxidative elimination of phenylselenous acid from the minor product afforded the allylic alcohol 124, whereas the major hydroxy selenide was resistant to oxidation and elimination. When 124 was treated with a small amount of acetic anhydride and sulfuric acid in acetic acid, the main product was the rearranged acetate 67, which had been previously converted to ( )-lycorine (108). 

The protected methyl glycoside 3 is converted to the corresponding aldehyde by Swern oxidation using oxalyl chloride activated DMSO. Further reaction with triethyl phosphonoacetate and sodium hydride -known as the Horner-Wadsworth-Emmons reaction - provides selectively the trans et /Tun saturated ester 4 in 72 % yield. This valuable alternative to the Wittig olefination protocol uses phosphonate esters as substrates which are readily available from alkyl halides and trialkyl phosphites via the Arbuzov rearrangement.9 co2Et Reaction of the phosphonate with a suitable base gives the... 

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