Rearrangement reaction with carbamates

The Curtius rearrangement provides a route to carbamates of a-amino sulfonamides (Scheme 25). 108 Reaction of the ethyl ester of an a-bromo acid 52 with Na2S03 yielded the sodium salt of the corresponding sulfonic acid 53. Treatment of 53 with PQ5 afforded the sulfonyl chloride 54 which on reaction with an amine gave the sulfonamide 55. The latter... 

We examined initially the photochemical reactivity of 2-ethoxypyrrolin-5-one (12) with olefins because of its obvious similarity to 2-cyclopentenone. Irradiation of 12 in the presence of 1,1-dimethoxyethene or cyclohexene in ferr-butyl alcohol solvent did not give a (2 + 2)-cycloadduct but the rearrangement product ferr-butyl N-(ethoxycyclopropyl)carbamate (I3)l4 The novelty and synthetic potential of the rearrangement of 12 to 13 lured us temporarily away from our original pursuit. However, as will become evident later, our studies of the rearrangement reaction were important in our eventual discovery of the (2 + 2)-photocycloaddition reaction. 

The starting point for the metallation sequence was /7-chlorophenol, whose directing power was first maximised by conversion to the carbamate 3. Ortholithiation and reaction with N,N-diethylcarbamoyl chloride gave the amide 4. The second ortho carbonyl substituent was then introduced simply by allowing the lithiated carbamate 5 to undergo an anionic ortho-Fries rearrangement to the bis-amide 6. The phenol was protected as its methyl ether 7. 

Allylic amines. Ally lie phenyl selenides on reaction with NCS (excess), an alkyl carbamate, and triethylamine at room temperature are converted into the corresponding rearranged carbamate-protected primary allylic amines. 

Primary allylic amines. Aziridines can be converted into allyl carbamates by reaction with ethyl chloroformate to form N-ethoxycarbonylaziridines followed by thermolysis in benzene at 200-250°, Rearrangement of trisubstituted ethoxycarbonylaziridines is regiospecific. 

Hofmann rearrangements Primary amides are converted into amines in high yield by reaction with this owdant at room temperature in aqueous acetonitrile (equation I). The reaction is rslated to a recent method using lead tetraacetate (6, 316) however, the intermet iate isocyanate in this case is not trapped as the carbamate, but is hydrolyzed alirectly to the amine. 

An asymmetric variant of this kind of allylic amination, based on their phenylcyclohexanol-derived chiral N-sulfinyl carbamates, was developed by Whitesell et al. (see also Sect. 3.2) (Scheme 34) [85]. After the asymmetric ene reaction with Z-configured olefins (not shown) had occurred, nearly di-astereomerically pure sulfinamides 127 were obtained which were found to be prone to epimerization. Their rapid conversion via O silylation and [2,3]-a rearrangement dehvered the carbamoylated allyhc amines 128 with around 7 1 diastereoselectivity as crystalline compounds that can be recrystallized to enhance their isomeric purity to 95 5. Obviously the imiform absolute configuration at Cl in the ene products 127 was difficult to transfer completely due to the already mentioned ease of epimerization. Unhke the sulfonamides of Delerit (Scheme 33) [84], the carbonyl moiety can easily be cleaved by base treatment. 

Radical reactions with organotin hydrides have been valuable for dehalogenation of organic halides. Bu3SnH/AIBN dehalogenation of a bicyclic carbamate furnished the ring-contracted product by carbamate rearrangement (Scheme 12.126) [229]. 

Recently, cyclizations of the isomeric unsaturated carbamates (141) and (142) have been investigated (equation 107). These fundamental reactions are not as regio- and stereo-selective as similar cyclizations starting from cyclic A -acyliminium ions (equations 25 and 26). The loss of stereoselectivity might have to do with the occurrence of cationic aza-Cope rearrangements in combination with chair-chair interconversions (equation 108). Normal cyclization of (142) occurs via conformation (145) and should lead to (144). If sigmatropic rearrangement competes with normal cyclization, (146) arises, but this conformation still leads to (144). Only after chair-chair interconversion to (147) and subsequent cyclization, (143) is formed. Via the same mechanism (141) can produce the abnormal product (144). 

The ene reaction (of, for example, the pinenes) with PhS02NS0 has been described, and the cyclization-induced [3,3] sigmatropic rearrangement of allylic carbamates e.g. linalyl A,A-dimethylcarbamate) catalysed by mercuric... 

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