Exocydic double bond

Knolker and coworkers also used a domino [3+2] cycloaddition for the clever formation of a bridged tetracyclic compound 4-172, starting from a cyclopentanone 4-168 and containing two exocydic double bonds in the a-positions (Scheme 4.36) [57]. The reaction of 4-168 with an excess of allylsilane 4-169 in the presence of the Lewis acid TiCLj led to the spiro compound 4-170 in a syn fashion. It follows a Wag-ner-Meerwein rearrangement to give a tertiary carbocation 4-171, which acts as an electrophile in an electrophilic aromatic substitution process. The final step is the... 

Cyclopentaf/z ][2.2.4]cyclazines (297), which are uncharged since there is an exocydic double bond at the [2.2.4]cyclazine periphery, have been prepared by the two routes shown in Scheme 46 <80JCS(Pl)l324, 79TL3405). Electrophilic substitutions (deuteration, nitration,... 

Benzo[Z>]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z>]furan and 2- and 3-vinylindoles give adducts involving the exocydic double bond. In contrast, the benzofc]-fused heterocycles function as highly reactive dienes in [4+2] cycloaddition reactions. Thus benzofc ]furan, isoindole (benzofc Jpyrrole) and... 

The N-basicity of the commonly used amines (pyrrolidine > piperidine > morpholine) drops by 2-3 orders of magnitude as a consequence of electron pair delocalization in the corresponding enamines. This effect is most pronounced in morpholino enamines (see table below). Furthermore there is a tendency of the five-membered ring to form an energetically favorable exocydic double bond. This causes a much higher reactivity of pyrrolidino enamines as compared to the piperidino analogues towards electrophiles (G.A. Cook, 1969). 

The base value for the a,p-unsaturated six-membered ring ketone system is 215 nm. Extended conjugation adds an additional 30 nm. The presence of an exocydic double bond, marked a, extends the another +5 nm. There is asubstituent on the p-carbon atom (+12 nm) and on the 8-carbon atom (+18 nm). There is no solvent effect because the spectrum was obtained in ethanol (0 shift).The total is 215 + 30 + 5 + 12 + 18 = 280 nm ... 

M = number of alkyl substituents in the conjugated system endo = number of rings with endocydic double bonds in the system i exo = number of rings with exocydic double bonds in the system... 

Treatment of 4-silylated -sultams 94 with aldehydes furnishes an exocydic double bond on the ) -sultam system (Scheme 2.57) [155]. The alkylidene-) -sultams 95 are obtained as mixtures of geometric isomers. 

Details of various routes to allylphosphine oxides (43) have been reported, and the subsequent synthesis of 1,3-dienes has been illustrated by many examples.37 Also described are stereochemical aspects of these diene syntheses and of subsequent Diels-Alder cycloaddition reactions.37 High regioselectivity is observed in migrations of the diphenylphosphinoyl group from unsymmetrical sites, as in (44), in that both products have a double bond exocydic to the cyclohexane ring.38... 

Another, more important conclusion concerns the general condition for the planarity of the enamine system. The scatter plot of XniI versus tNi c] suggests that the planar arrangement of the enamine core atoms is most probable when and/or the double bond C =C are exocydic. 

From among the two geometries available to 43, the choices are to orient the cyclopentenyl unit as in M or N. In the first instance, since a chair-hke conformation shall develop, a Z double bond will be generated, and H-5 will be projected to the a-face. On the other hand, another conformation N, which requires a boat-Uke geometry, results in the evolution of an E double bond, and fixes H-5 in a j8-orien-tation. In addition, N suffers from steric compression between the exocydic methylene group and the silyl substituent. On the basis of these many factors, transition state M should be kineticaUy favored. 

---