Retrosynthetic analysis corresponding to synthesis in Scheme

Scheme 13.27. Retrosynthetic Analysis Corresponding to Synthesis in Scheme 13.28... 

Longifolene synthesis E. J. Corey, R. B. Mitra, and P. A. Vatakencherry Longifolene synthesis J. E. McMurry and S. J. Isser Retrosynthetic analysis corresponding to synthesis in Scheme 13.19 Longifolene synthesis R. A. Volkmann, G. C. Andrews, and W. S. Johnson... 

A retrosynthetic analysis corresponding to the synthesis in Scheme 13.28 is given in Scheme 13.27. The striking feature of this synthesis is the structural simplicity of the key intermediate 27-IV. A synthesis according to this scheme generates the tricyclic skeleton in a single step from a monocyclic intermediate. The disconnection 27-III-27-IV corresponds to a cationic cyclization of the highly symmetric allylic cation 27-IVa. 

The first stereocontrolled syntheses of juvabione are described in Schemes 13.11 and 13.12. Scheme 13.10 is a retrosynthetic analysis corresponding to these syntheses. These syntheses have certain similarities. Both start with cyclohexenone. There is a general similarity in the fragments that were utilized, but the order of construction differs. In the synthesis shown in Scheme 13.11, the crucial step for stereochemical control is step B. The first intermediate is constructed by a [2 + 2] cycloaddition between reagents of complementary polarity, the electron-rich enamine and the electron-poor enone. The cyclobutane ring is then opened in a process which corresponds to retrosynthetic step Ha => Ilia in... 

The first successful synthesis of longifolene was described in detail by E. J. Corey and co-workers in 1964. Scheme 13.19 presents a retrosynthetic analysis corresponding to this route. A key disconnection is made on going from I => II. This transformation simplifies the tricyclic skeleton to a bicyclic one. For this disconnection to correspond to a reasonable synthetic step, the functionality in the intermediate to be cyclized must engender mutual reactivity between C-7 and C-10. This is achieved in diketone II, because an enolate generated by deprotonation at C-10 can undergo an intramolecular Michael addition to C-... 

Explain what is happening in this reaction. Draw a scheme of retrosynthetic analysis corresponding to the synthesis. How would you make the starting materials ... 

Applications of the same enolate alkylation procedure to furnish gem-dialkylated compounds have been used in the synthesis of onnamide A [118]. This type of gem-2-C-dimethyl group is also found in pederin and mycalamides A and B. As shown from the retrosynthetic analysis of some members of this class of compounds (Scheme 11.38), C-glycosidic structure 158 could be obtained from the a-glycoside 159. The latter could be prepared from the ketone 160 via gem-dialkylation of the corresponding enolate, as described for the synthesis of 8. 

In many cases, fragmentations corresponding to the retroreaction of a powerful reaction (sec. lO.S.B.vi) can be identified and used retrosynthetically. Retro-mass spectral analysis has been applied to total synthesis by Kametani. The mass spectra of tetrahydroisoquinoline alkaloids show fragmentations by the three major pathways shown in Scheme 10.24.1 Kametani observed that path c (a retro Diels-Alder reaction) was the major fragmentation pathway (sec. ll.S.B). Xylopinene (274) gave this specific fragmentation (to 275 and... 

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