Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0481 544

O

B OPh

O

(93)

Diastereoselectivities of up to 90% were observed for the cycloadditions of N-galactosy- limines 108 (Piv D pivalyl; R D 2-furyl, 2-thienyl-, 4-FC6H4, 4-ClC6H4 or 3-pyridyl) to isoprene in the presence of zinc chloride to form the tetrahydropyridines 109 and 110 (equation 62)55.

The enantioselective (76–90% ee) formation of the dihydro- -pyridones 113 from various imines 112 (R D Ph, 3,5-xylyl or 3-pyridyl) and Danishefsky’s diene 4 in the presence of 4 A˚ molecular sieves and one equivalent of a catalyst prepared from triphenyl

borate or a trialkyl borate and (R)-Binaphthol 111 in dichloromethane has been reported (equation 63)56. Similarly, the chiral zirconium complex 114 (L D 1-methylimidazole) catalyzes the reaction of the diene 4 with the Schiff’s base 115 in toluene at 45 °C to yield, after hydrolysis, 88% of the optically active dihydropyridone 116 in 90% ee (equation 64)57.

Me

In a study of the Lewis-acid catalyzed formation of optically active dihydro- -pyridones 118 from the imines 117 (R D Ts, Ph, Bn or CO2Et) and the diene 4 in the presence of chiral Lewis acids, it was found that only the tosyl compound reacted diastereoselectively,

giving the product in 68% yield and 80% de (equation 65)58. In the presence of 0.1 equivalent of diethylaluminum chloride, cyclopentadiene adds the chiral imine 119 to give a mixture of the diastereomers 120 and 121 in the ratio 12 : 88 (equation 66)59.

OH

OH

(111)

Imines 123, generated from ˛-arylethylamines 122 (Ar D Ph, 4-BrC6H4 or 4-O2NC6H4) and aqueous formaldehyde, react with cyclopentadiene in situ at room temperature to afford mixtures of the bridged dihydropyridines 124 and 125 (equation 67), whose relative configurations were deduced by 1H NMR experiments and their absolute configurations assigned by reference to the X-ray structure of the aziridinium derivative 126 (equation 68)60.

Me

(126)

Protonated imines are effective dienophiles. Thus in the reaction of methyl glyoxylate with the hydrochloride 127 of alanine methyl ester in the presence of cyclopentadiene, a mixture of hydrochlorides of the exo- and endo-adducts 128–131 was formed (equation 69). The diastereomeric ratio of the exo-compounds was 83 : 1761.

The iminium salt 132, generated from benzylamine hydrochloride and aqueous formaldehyde, reacts with cyclopentadiene during 3 h at room temperature to give, after basification, the cycloadduct 133 in nearly quantitative yield (equation 70). Other examples of this reaction are shown in equations 71–75. The separable diastereomers 134 and 135 are formed in the ratio 4 : 1 from cyclopentadiene, ( )-˛-methylbenzylamine hydrochloride and aqueous formaldehyde in a combined yield of 86% (equation 75)62. Hydrochlorides 136 of methyl esters of natural amino acids [(S)-valine, (S)-isoleucine] react with cyclopentadiene and formaldehyde in aqueous THF to produce mixtures of the diastereomers 137 and 138, in which the former predominate (equation 76)63.

1-Azirines are reactive dienophiles64. The cycloadducts to cyclopentadienones spontaneously extrude carbon monoxide and undergo opening of the three-membered ring, followed by a 1,5-shift of hydrogen, to yield 3H-azepines (equation 77)65,66.

Treatment of the azetidinone 139 with zinc chloride generates the highly unstable azetinone 140, which is trapped as the carbacephem 142 in the presence of the diene