2.4- Dienyl carbamates

The isomer (Z,Z)-319 furnished under identical conditions, surprisingly, the (15, 2Z,7Z)-cyclonona-2,7-dienyl carbamate 323 with 88% ee, which could be converted to the free alcohol 324 (equation 86). The stereochemical outcome implies inversion of the configuration at the carbanionic centre in (S)-322. The method could also be successfully applied to 5-oxy-substituted dienes. ... 

Z)-awh-4-Hydroxy-l-aIkenyl carbamates 363, when subjected to substrate-directed, vanadyl-catalysed epoxidation , lead to diastereomerically pure epoxides of type 364 (equation 99)247,252,269 qqjggg epoxides are highly reactive in the presence of Lewis or Brpnsted acids to form -hydroxylactol ethers 366 in some cases the intermediate lactol carbamates 365 could be isolated . However, most epoxides 364 survive purification by silica gel chromatography . The asymmetric homoaldol reaction, coupled with directed epoxidation, and solvolysis rapidly leads to high stereochemical complexity. Some examples are collected in equation 99. The furanosides 368 and 370, readily available from (/f)-0-benzyl lactaldehyde via the corresponding enol carbamates 367 and 369, respectively, have been employed in a short synthesis of the key intermediates of the Kinoshita rifamycin S synthesis . 1,5-Dienyl carbamates such as 371, obtained from 2-substituted enals, provide a facile access to branched carbohydrate analogues . 

Surprisingly, RuX2PR3(r 6-arene) complexes did not promote the addition of ammonium N,N-dialkyl carbamates to alkenylacetylenes. However, this reaction was catalyzed by tr-allyl ruthenium derivatives such as [Ph(CH2) PPh2]Ru(r 3-CH2=C(Me)CH2)2 (n = 1-4), and yielded 0-l-(l,3-dienyl)carbamates (4—62% yield)... 

In this asymmetric Heck reaction, a quaternary carbon center is formed providing a tricyclic dienyl carbamate after reductive elimination of palladium. 

Initial experiments showed that the asymmetric Heck cyclization of 13 can be realized with several chiral enantiopure ligands. For this transformation, the Pfaltz ligand 14 proved optimal providing the dienyl carbamate 15 in an enantioselectivity of 99 % ee. This Heck cyclization is slow under traditional heating (requiring more than 70 h at 100 °C) but can be accomplished in 30 min with no decrease of enantioselectivity at 170 °C in a microwave reactor. Addition of an excess of trifluoroacetic acid to the crude product furnishes (dihydro-iminoethano)carbazole 15 in 75 % yield over two steps. 

The key step of this synthesis is a sequential asymmetric Heck-N-acyliminium ion cyclization of dienyl carbamate triflate 13 to provide enantiopure 3,4-dihydro-9a,4a-(iminoethano)-9/f-carbazole 16. This intermediate and related structures containing a 2,3-double bond should represent versatile precursors for constructing a variety of pentacyclic indole alkaloids containing the (hydroiminoethano)-carba-zole fragment. 

In another beautiful application, Overman et al. developed the total synthesis of the natural product (+)-minfiensine (130), having a l,2,3,4,-tetrahydro-9a,4a-(iminoethano)9H-carbazol core (Scheme 7.29) [80]. The key step in their synthesis is a tandem process consisting of a catalytic asymmetric Mizoroki-Heck ring closure and an N-acyliminium ion cyclization of dienyl carbamate triflate 127 (127—>128). The reaction proceeded smoothly using the Pfaltz ligand (S)- Bu-PHOX (85), providing (dihydroiminoethano)carbazole 129 after the addition of excess trifluoroacetic acid, with 75% overall yield and 99% enantiomeric purity (127 129). 

A methacrylic acid (MAA)-divinylbenzene (DVB) copolymer was noncovalently imprinted with TSAs for the endo- and exo-reaction pathways (5 mol%). The TSAs were prepared by an analogous Diels-Alder reaction between cyclohexa-l,3-dienyl-carbamic acid benzyl ester and N,N-dimethylacrylamide (Scheme 9b). It should be remembered... 

As indicated in Section ni.B, deprotonation of a carbamate affords a dipole-stabilized a-amino-organolithium that can be transmetalated with copper salts to form cuprates, thereby expanding the versatility of the organolithium. Suitable electrophiles include enones, alkenyl, alkynyl, allenyl and dienyl carboxylic acid derivatives, nitriles and sulfoxides. Dieter and coworkers have shown that the same process can be accomplished via transmetalation of a stannane (Scheme 36). The procedure is particularly... 

The introduction of the aminoethyl side chain is accomplished by Suzuki cross-coupling of dienyl triflate 10 with the alkyl borane generated by hydroboration of A -vinyl- er butyl carbamate (11) with 9-BBN. ... 

Scheme Jll Alkyl dienyl carbonates and carbamates from reaction ofcrotonal-dehydes with KOtBu and acyl chlorides.

In related work, sixteen-fold deprotonated Y yclodextrin f(Hms toroidal complexes with hexanuclear lead(II) of empirical formula [Pb 6(T CDH.i6)2l.ca.20H2O, and with a structure consisting of a ring of "inside and a ring of "outside" lead atoms, 60.129 The structure of a 3,4-mannitolate bis(phosphine)platinamdienyl complex 62 02 has been described. 

Amides and carbamates undergo analogous alkylation reactions with dienyl carbonium salts (A. J. Liepa, personal communication) as do purines and pyrimidines (I. D. Jenkins, unpublished). Another useful application of dienyl carbonium salts is in the synthesis of 2,4-dienylphosphonic acid derivatives through a facile Arbusov reaction with phosphites [Eqs. (114) and (115) (Birch et a ., 1975)]. This type of compound would be difficult to synthesize by normal procedures. 

Vol. 28, p. 226, ref. 44 for related work). Similar cycloadditions of dienyl pyrrolidinones [le. AT-(peiita-l,3-dienyl)- or -(buta-l,3-dienyl)-pyrolidin-2-one] with the in situ generated acyl nitroso compound derived from benzyl-JV-hydroxy-carbamate with periodate have led to the preparations of racemic pyroUidines 56 and 57 (R = H or Me). The latter compound is thought to exist as a dimer. An asymmetric synthesis of l,S,6-trideoxy-l,5-imino-D-altritol in which the piperidine ring is formed from a pyiidinium ring bearing Seebach s oxazolidinone chiral auxiliary has also been described. ... 

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