Unsaturated imines

The Pd(0)-catalyzed rearrangement of the iV-allylenamine 800 in CF3CO2H affords the (5, -unsaturated imine 801, which is hydrolyzed to give the 7, 8-unsaturated aldehyde 802[498]. The vinyloxaspirohexane 803 undergoes rearrangement-ring expansion to give the cyclopentanone 804 in the presence of 1 equiv. of p-nitrophenol[499]. 

Imines with an electron-withdrawing group at the nitrogen atom are excellent acceptors for the acetate (1) or the carbonate (13) [36]. Thus, N-tosylimines (84) gave very good yields of pyrrolidines (85) under typical conditions. The strained cyclic imine (86) and a,/ -unsaturated imine (87) both participated smoothly in the cydoadditions. The hindered nitrimine (88) also reacts well with (1) (but not with 13) to produce the pyrrolidine (89) with a 17 1 diastereoselectivity. However, the unhindered nitrimines from cyclohexanone and 2-nonanone failed to react presumably due to enolization (Scheme 2.24). 

The amino acid methionine is biosynthesized by a multistep roule that includes reaction of an inline of pyridoxal phosphate (PLP) to give an unsaturated imine. which then reacts with cysteine. What kinds of reactions are occurring in the two steps ... 

An efficient procedure for the synthesis of 2,4,6-trisubstituted and 2,3.4,6-tetrasubstituted pyridines 5 and 6 involves the one-pot reaction of in situ generated a,p-unsaturated imines with carbanions <95TL(36)9297>. 

A convenient regioselective one-pot approach to pyrazolo[l,5-o]- and imidazolo[l,2-a]pyrimidine derivatives 79 from aminoheterocycles 78 and a,(3-unsaturated imines 77,... 

The palladium-catalyzed hetero-[4 + 3]-cycloadditions reported by Trost and Marrs utilize a metal-complexed trimethylenemethane as the three-carbon component. These complexes react with a,/3-unsaturated imines to produce seven-membered heterocycles in moderate to good yields.84 Two examples of this reaction were reported and are shown in Equations (13) and (14). Only the [4 + 3]-reaction was observed with a,/3-unsaturated imine 76 however, both the [4 + 3]- and the [3 + 2]-modes of reactivity are observed with a,/3-unsaturated imine 79. 

Various substituted /J,y-unsaturated imines (equation 77) did undergo aza-di- r-methane... 

The reaction is assumed to occur via the formation and cyclization of a ketyl, 141, followed by the addition of a second electron and two protons, leading to an unstable a-hydroxyimine 142. Hydrolysis affords the ketol 139, while elimination of water, followed by a second reduction of the resulting a,)S-unsaturated imine and hydrolysis, leads to the ketone 140. 

Originally it was considered that one of the main attractive features of the 1-ADPM rearrangement of (3,7-unsaturated imines, from a synthetic point of view, was that it could serve as a surrogate for the conversion of p,7-unsaturated... 

Addition of organolithium reagents to double bonds 2. Addition to a,8-unsaturated imines... 

The sense of the enantiofacial selection was predictable from the model complex of organolithium, imine and chiral diether 28, where the migrating C—Li bond is parallel to the 7T-system of the a,/3-unsaturated imine (Figure 4). From the favored complex the R group of the organolithium reagent is transferred to the less hindered face of the double bond of the unsaturated imine. 

Here we discuss several examples of the multicomponent processes involving aminoazoles, aldehydes, and other organic components such as mercaptoacids, haloacetic acids and their ester, a,p-unsaturated imines, etc., which were not incorporated into the previous two sections of the review. 

In addition, an interesting treatment of in situ-generated a,p-unsaturated imines with aminoazoles was described in [150]. This one-pot procedure yielded the anticipated pyrazolo[l,5-a]- and imidazo[l,2-a]pyrimidine derivates 112 (Scheme 52). 

Scheme 52 MCRs proceeding via unsaturated imines formation...

Scheme 53 Mechanism of the MCRs via formation of unsaturated imines...

A few pyrolytic methods of synthesis are known, leading directly to the formation of pyridine rings by formation of the /3, y-bond. Alkylpyridines are obtained when unsaturated imines (116) or (117) are passed over heated zeolites (80IZV655) or alumina (72IZV2263). More dehydrogenation is achieved by the use of nickel or alumina, as in the synthesis of tetrahydroquinolines (118) or octahydrophenanthridine (119) (78IZV1446). 

The electrophilic cyclization of unsaturated imines derived from glyoxalate, catalyzed by ytterbium triflate, gives reduced furo[3,4- ]pyridines in variable yields, but with good stereoselectivity (Equation 16) <2002TL2339>. For the 5 6 ring system, the /ra t-isomer, 73, is the primary product formed. 

---