Boat-like

A highly successful route to stereoisomers of substituted 3-cyclohexene-l-carboxylates runs via Ireland-Claisen rearrangements of silyl enolates of oj-vinyl lactones. The rearrangement proceeds stereospeaifically through the only possible boat-like transition state, in which the connecting carbon atoms come close enough (S. Danishefsky, 1980 see also section 4.8.3, M. Nakatsuka, 1990). 

In 1997, Lindstrom and Somfai reported aza-[3,3]-Claisen enolate rearrangements of vinylaziridines (Scheme 2.45) [70]. Treatment of l-acyl-2-vinylaziridines 179 with LHMDS resulted in the stereoselective formation of seven-membered lactams 181, presumably through a boat-like transition state 180. 

The stereochemistry of this reaction is consistent with transition state 2 in which the ethoxycar-bonyl unit adopts an equatorial position. The same result could occur, however, via boat-like transition state 3 with an axial ethoxycarbonyl group47. The reactions of 2-oxopropanoate esters and 9-(2-butenyl)-9-borabicyclo[3.3.1]nonane occur at — 78°C, reflecting the greater reactivity, but stereoselectivity is generally poor except in cases where a very hindered ester is employed3811. 

The preferential conversion of the aluminum enolatc 2 c to diastereomer 3 is consistent with the boat-like transition state A where the enolate has adopted the usual antiperiplanar E geome-try42-50. The aldehyde substituent occupies the lower energy equatorial position of a boat-like transition state which places the bulky dialkylaluminum moiety away from the iron ligands. Possible transition states for the other observed diastereomeric products are also illustrated. 

In the case of an aromatic 7t-nucleophile, a dramatic reversal of diastercoscleetivity is found on going from a phenyl to a cyclohexyl or fert-butyl substituent, which is attributed to a balanee of. syn-axial and A(1,3) interactions, while a boat-like transition state may also play a role149. 

A c/.v-fused chair-chair -like and a Tram-fused boat-boat -like transition state have been suggested to explain the stereochemical outcome from the addition reactions of the (E)- and (Z)-l-(/f>rf-butylsulfinyl)-2-methyl-2-butenes, respectively. 

In the rearrangement of divinylaziridines 289, the participation of a boat-like transition state 290 explained the stereochemical outcome of the reactions to give the azepinones 291 in 73 to 85% yield. The divinylaziridines 289 were synthesized via ex-chiral pool sequences starting from optically active a-amino acids. Table 16, Eq. (26) [55]. 

Instead, transannular strain between the silylether and one of the Cp ligands leads to a product 77 from a boat-like insertion transition structure (76 bottom). 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

We also performed a single-crystal X-ray structure analysis of this lead compound. The solid state structure of this compound depicted in Fig. 3-15 shows a half-boat-like ( sofa ) conformation with the 9-phenanthryl group in a quasi-axial or r/Mf/.v/-flagpole position, and the a, 3-unsaturated exocyclic ester in a s-cis conformation. This cleft-like conformation is advantageous for the creation of centers with a high recognition ability, since one enantiomer fits in better than the other thus leading to selectivity. 

Thus a Cope rearrangement proceeding through a boat-like transition state is the rearrangement of cis 1, 2 divinyl cyclo-propane. 

In the transition state a boat like structure appears and there will be a suprafacial cis addition to the termini of the n bond. The ene reaction does not have a symmetrical transition state and it is a thermally allowed concerted reaction. Its transition state involves a suprafacial interaction of six electrons (4 from the k bonds and two from the o bond) So it is a Huckel system and transition state is aromatic. In the terminatlogy of Woodward and Hoffmann it can be regarded as o2s + n2s + 7t2s reaction. 

Rearranging divinylaziridines 95, the passing of a boat-like transition state... 

Surprisingly, the rearrangement of the 4-f-butyldimethylsilyloxy-2-vinyl-pyrrolidines 154 (R =OTBS, R R =H) took another course. The stereochemical outcome had to be rationalized by the passing of a boat-like transition state 156 (bj8) to give the 3,8-trans lactams 157 (Ri=OTBS, entries 15-19, Table 8). The corresponding cis product 158 (Ri=OTBS) resulting from the expected chairlike intermediate 156 (c ) had only once been isolated as a minor compound... 

Claisen rearrangement presumably was the boat-like form 156 (bj0) with minimized 1,3 repulsive interactions resulting in the lactams 157. However, the 2,4-as disubstituted pyrrolidine 159 (R =OTBS, R, R =H) gave the expected lactam diastereomer 158 via a chair-like transition state conformation 160 (entry 16, Table 8) (Scheme 31). 

The rearrangement of E and Z N-crotylamines 245 (R, E or Z=Me) gave the corresponding nitriles 247 with 82 and 68% yield, respectively. Disappointingly, the product was obtained as an inseparable mixture of synlanti diastereomers 247 indicating a low simple diastereoselectivity. Obviously, the intermediate ketene imine fitted neither a chair- nor a boat-like conformation. Hence, a low axis-to-center chirality induction was operative, and E and Z reactants gave a... 

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