Extrusion ring method

System Base induced elimination FVP Dehalogenation Desilylation Related methods SO2 extrusion CO2 extrusion Ring opening of cvciobutene... 

This group has also developed two ring-contraction systems of potential use in crown synthesis. In the first of these, extrusion of a phenylphosphine oxide unit results from treatment with alkoxide ion. In the second, similar conditions initiated decarbonyla-tion of a bis-pyridyl ketone Despite the apparent potential of these methods for crown synthesis, direct formation of crowns by processes which involve them do not appear to have enjoyed great success thus far. 

The success of the above alkylation and acylations, without obtaining ring-opening products , extends the usefulness of this method particularly when the anion 323 is being used in a regio- and stereo-specific manner . Thus, the combination of direct alkylation and thermal extrusion of sulfur dioxide provides an ideal route for the preparation of terminally substituted conjugated dienes. 

As mentioned on pages 317 and 324, the 1,3-dipolar electrocycUzation of thiocarbonyl ylides leads to thiirane derivatives, which represents an excellent method for the preparation of those three-membered rings. Typically, thiiranes are isolated as the final products, but in some instances they are produced as intermediate compounds which spontaneously desulfurize to give alkenes [twofold extrusion (47,48)]. 

Diazo compounds also undergo cycloaddition with fullerenes [for reviews, see (104),(105)]. These reactions are HOMO(dipole)-LUMO(fullerene) controlled. The initial A -pyrazoline 42 can only be isolated from the reaction of diazomethane with [60]fullerene (106) (Scheme 8.12) or higher substituted derivatives of Ceo (107). Loss of N2 from the thermally labile 42 resulted in the formation of the 6,5-open 1,2-methanofullerene (43) (106). On the other hand, photolysis produced a 4 3 mixture of 43 and the 6,6-closed methanofullerene (44) (108). The three isomeric pyrazolines obtained from the reaction of [70]fullerene and diazomethane behaved analogously (109). With all other diazo compounds so far explored, no pyrazoline ring was isolated and instead the methanofullerenes were obtained directly. As a typical example, the reaction of Cgo with ethyl diazoacetate yielded a mixture of two 6,5-open diastereoisomers 45 and 46 as well as the 6,6-closed adduct 47 (110). In contrast to the parent compound 43, the ester-substituted structures 45 and 46, which are formed under kinetic control, could be thermally isomerized into 47. The fomation of multiple CPh2 adducts from the reaction of Ceo and diazodiphenylmethane was also observed (111). The mechanistic pathway that involves the extrusion of N2 from pyrazolino-fused [60]fullerenes has been investigated using theoretical methods (112). 

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

Another route to corrin via 4,5-seco intermediates has been established (Scheme 88).250,255 It was shown by X-ray structure analyses of the Ni-4,5-secocorrin, and its cyclization product, that the pyrrolic A and B rings are properly oriented for the reaction.250 A sulfur extrusion method was employed in the B12 synthesis of Woodward and Eschenmoser.253... 

The synthetic pathways leading to tetradehydrocorrins and isobacteriochlorins are very similar and it is just by fine variations of the reaction conditions that the preparation is driven towards specific tetrapyrrolic rings. The linear precursors have been synthesized using the sulfide contraction method (also indicated by other authors as sulfur extrusion ). The corrin skeleton is formed by alkaline hydrolysis of the cyano protecting group present at the 19 position and subsequent acid catalyzed coupling of pyrroles A and D, as described in Fig. 26. 

Extrusion of sulfur dioxide from a ring-fused dihydiothiophene derivative provides an exceptionally easy method for the preparation of heterocyclic o-dimethylene compounds. These compounds are valuable intermediates in intermolecular Diels-Alder reactions. This extrusion method was used to prepare the o-dimethylene compounds 44-46 <95CC1349> as well as quinolinone derivative 47 <95TL5983>. TTie dihydrothiophene dioxide moiety also played an important role in the formation of the intramolecular Diels-Alder reaction of N-substituted pyrrole 48 <95CC807>. 

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