Acyclic carbamates

Rearrangement of carbamate roc-13aa, which carries the least sterically demanding methyl group at the N atom, proceeded the most efficiently in terms of 

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Bicyclic structures of this type are more reactive than monocyclic or acyclic carbamates, indicating that a relatively rigid orientation of the carbonyl group is favorable to lithiation. Substituted formamidines can also be lithiated.60... 

Scheme 6.3S. Interpretation of the chirality transfer during the course of allylic substitution of acyclic carbamate derivative (R)-163.

Whereas the Hofmann rearrangement of the amidoalcohols 452 with the usual NaOCl did not occur, reaction of the oxygen-bridged 452 (X = 0) with bis(acetoxy)iodobenzene under mild conditions led to a 9 1 mixture of the corresponding cyclic and acyclic carbamate derivatives 197 and 453 (X = 0) (Equation 50). In the similar reaction of the methylene-bridged analog 452 (X = CH2), the product of perhydro-3,l-benzoxazin-2-one type 197 (X = CH2) was obtained exclusively <20030PP429>. 

Enamides are excellent substrates for Vilsmeier reactions, possibly because of their lower reactivity. The acyclic carbamate 44 gives the acrylaldehyde in high yield, but stereochemical integrity is lost (Eq. 36)." As in many Vilsmeier reac-... 

A similar transformation was subsequently reported in 2014 by Fu and coworkers, who used allene and carbamate precursors to generate [3] dendralenes via rhodium(III) catalysis (Scheme 1.34) [158]. A variety of different carbamates 213 successfully rmderwent Rh(III)-catalyzed C-H activation and coupling to generate cychc and acyclic substituted [3] dendralenes 214. While the general route works quite well between the tri-substituted allene and acyclic carbamates, the reaction is not high yielding if the enol ester is cychc or the allene is mono- or tetra-substituted. 

Another occasionally used method for the preparation of acyclic A-(l-alkoxyalkyl)amides (or carbamates) proceeds via addition of reactive carboxylic acid derivatives to aldimines. In a one-pot procedure, treatment of the imine with the acid chloride (or ethyl chloroformate) and subsequent (m)ethanolysisofthe intermediary a-chloroamide leads to the oc-alkoxyamide56-58. 

The electrochemical oxidation of cyclic and acyclic, V-monosubstitilted and ATY-disubstituted amides and carbamates in a nucleophilic solvent, known as the Ross-Eberson-Ny berg reaction, is a synthetically very useful, clean and efficient method for the introduction of a-oxygen substituents under mild reaction conditions6 1 0. 

The intermolecular coupling of lactams and acyclic amides has also been reported. Reactions of carbamates with aryl halides occurred in the presence of catalysts ligated by P(/-Bu)3.78 Both carbamates and amides coupled with aryl halides in the presence of a catalyst bearing Xantphos.90 In addition, the coupling of lactams with aryl halides has been successful. A combination of Pd(OAc)2 and DPPF first formed A-aryl lactams in good yields from 7-lactams, but the arylation of amides was improved significantly by the use of Xantphos (Equations (20) and (21)).90 91 The reaction of aryl halides with vinyligous amides has also been reported 92... 

In contrast, reactions catalyzed by la were typically conducted with added [Ir (C0D)C1]2 to trap the K -phosphoramidite ligand after dissociation, and thereby, to leave the unsaturated active catalyst. Under these conductions, as much as half of the iridium in the system is present in an inactive acyclic species. In contrast, when ethylene adduct lb is used as the catalyst, all of the iridium belongs to the active metalacyclic species. Hartwig and coworkers have recently taken advantage of the increased availability of the active catalyst generated from lb to develop new allylic substitution reactions. These new processes include the reactions of carbamates, nitrogen heterocycles, and ammonia. 

Recently, Scialdone and colleagues have employed phosgenated />-nitrophenyl(poly-styrene) ketoxime (Phoxime, 76) resin in the synthesis (via 77 and 78) of acyclic and heterocyclic amino-acid-derived ureas 79 too (Scheme 42). Resin 75 was first reacted with an amino acid and TMSCl in pyridine, and the resulting carbamate acid resin 77 was then coupled with an amine under standard carbodiimide protocol (HOBt/DIC in DMF) to obtain the carbamate amide resin 79. 

Surprisingly it was reported that when potassium cyanate is substituted for sodium cyanate the yields of carbamates are reduced to less than 5 %. The reason for this drastic effect is not known at this time. In addition, the use of other alkali or alkaline metal cyanates in this reaction has not been investigated. The Loev [28] procedure appears applicable to the synthesis of carbamates from primary, secondary, and tertiary alcohols (2 hr reaction time affords 60-90% yields), cyclic and acyclic 1,3-diols, phenols, oximes, ald-oximes, and ketoximes, and primary, secondary, and tertiary mercaptans. Carbamates could not be obtained from diphenylethylcarbinol (dehydrated to 1,1-diphenylethylene) or trichloro- and trifluoromethylcarbinols. 

Acyclic ureas have been obtained in moderate yield (up to 66%, after 20 h) by the thermal decomposition of carbamate salts (RNH3)02CNHR at high temperature (453 K), without the use of a catalyst [127a, b]. At high temperatures and pressures, diamines can also react with C02, in the absence of a catalyst, to afford cyclic ureas (imidazolidin-2-ones, tetrahydro-pyrimidin-2-ones) in moderate to good yield and selectivity [69c, 127c]. 

The alcoholysis of urea affords stepwise acyclic carbonates through the formation of an alkyl carbamate intermediate (Scheme 7.2). The reaction of urea with alcohols to carbamate is exothermic, whereas the subsequent reaction-carbamate to carbonate-is endothermic. Thus, the ideal gas free energy, AG, is positive for the latter step, which means that a low yield of carbonates would be expected [12],... 

Acyclic C-N bonds of allyl or benzyl triflamides,51 pivalamides and carbamate derivatives9 can be reductively lithiated, as can N-benzyl benzotriazoles.72... 

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