Ring closure reactions, synthetic strategy

Scheme 2-7. Synthetic strategy of ring closure reactions according to Evans et al. ...

This synthetic strategy also proved to be also suitable for the ring closure to the related [l,3,4]triazolo[2,3-a][l,3,5]tri-azinium salts, as shown in Scheme 33 <1994JHC535, 1995JHC1833> reaction of 169 with l-chloro-l,3-bis(dimethyl-amino)-3-phenyl-2-azaprop-2-enylium perchlorate gave the quaternary salt 170 in 37% yield. 

After the preparation of the 2-methylcysteine compounds, the ring closure to the thiazoline 33 is the next step in most of the synthetic strategies. The key reactions used are summarized in Scheme 9. 

To build the central ABC core present in 1, two main synthetic strategies have been used Diels-Alder reactions and asymmetric synthesis. This tricyclic subunit was chosen as the first synthetic target because it contains all five stereogenic carbons. A central pyrrolo-isoquinoline skeleton (ABC) was initially synthesized by the high-pressure Diels-Alder reaction of 3-alkyl-5,6-dihydro-2-pyridones with Danishefsky s diene, followed by deprotection and spontaneous pyrrolidine ring closure (see Scheme 16) [77]. 

Other synthetic strategies based on cyclisation of ring B involve synthesis of an intermediate DCA entity through anionic reactions followed by ring closure under Friedel-Crafts conditions. Thus Michael addition of the nitrile 34 to the... 

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