Reaction between 3H-pyrrolizines and acetylenedicarboxylic esters. Part III. The photochemical reaction

Heterocycles Containing a Ring‐Junction Nitrogen

“…In the parent compound 1 (R ¼ H) the 1H-tautomer could not been detected in the 1 H NMR spectrum. However, in the attempted preparation of 5-methyl-3H-pyrrolizine (R ¼ 5-Me) the formation of a mixture of 1a, 1c, and 1d in a 44 : 66 : 10 ratio was observed [22]. In pyrrolizines substituted in the C2(C6) position, the tautomeric equilibrium depends on the electronic nature of the substituent.…”

“…In pyrrolizines substituted in the C2(C6) position, the tautomeric equilibrium depends on the electronic nature of the substituent. For example, tautomer 1b is the stable form for 2-methyl-and 2-phenylpyrrolizine [22,23] whereas when R ¼ 2-COPh or 2-CO 2 Me the tautomeric form 1c is more stable [24]. In terms of reactivity, 3H-pyrrolizines are vinylpyrroles and a predominant reactivity towards electrophiles can be expected.…”

“…Derivative 98 can also be brominated by Nbromosuccinimide (NBS) under free radical conditions to give 2-bromopyrrolizin-3-one (102) (Scheme 22.12) [47]. Wittig reactions on the parent 98 to give azafulvenes such as 101 are also known [22]. …”

“…The addition of the carbene to 3H-pyrrolizine gives a 3-pyrrolizinyl carbene 88 and subsequent addition to the C1-C2 bond affords the tetracyclic pyrrolizine derivative 89, which evolves to indolizine (7) [43] by a ring expansion process (Scheme 22.9). Tricyclic derivatives 90 and 91 are obtained in low yield by [2 þ 2] cycloadditions with DMAD and dichloroketene generated from trichloroacteyl chloride and activated zinc in ether, respectively [22,44].…”

“…Figure 22.1 shows some of the general structures for combinations of five-and sixmembered rings. (16), quinolizines (17)(18)(19), quinolizidine (20) and azaquinolizinium isomers (21)(22)(23)(24); and (d) benzoquinolizinium salts and related systems.…”