Fragmentation-type ring

As practiced in the preceding syntheses by Evans and Nishiyama and Yamamura, the A-ring fragment 43 is formed through substrate-directed vinylogous aldol reaction of the Brassard-type diene 19 and the chiral aldehyde 42, which is prepared using Brown s protocols for enantioselective allylation [53], followed by hydroxy-directed nnn-diastereoselective reduction of the C3 ketone (Me4NB(OAc)3H) [41],... 

Two unique type Had syntheses of pyrroles that were reported both involved cyclopropane fragmentations. The first allowed for a synthesis of 2-arylpyrroles <06SL2339>. In the event, treatment of stannylcyclopropane 25 with -BuLi followed by benzonitrile produced 2-phenylpyrrole 26 via tin-lithium exchange, addition to the nitrile, ring fragmentation of ketimine intermediate, intramolecular condensation, and loss of dibenzylamine. 

The presence of the cyclic backbone in ligands of this type makes a substantial contribution to their metal-ion complexing ability even though coordination involves donors which are not directly incorporated in the ring fragment. The origins of the enhanced stability of the metal-containing species may be considered to reflect the operation of an indirect macrocyclic effect (see Chapter 6) in these systems. 

Isoxazoles and benzisoxazoles that are unsubstituted at the 3-position readily undergo the base-reduced ring fragmentation shown above, and there are therefore no reports on the successful metalation of these types of compound. 3-substituted isoxazoles do undergo lithiation, at the 5-position, but ring fragmentation rapidly follows, even at -60°C in the case... 

The mass spectra of diverse types of 1,3-thiazines are reported in the literature and in most examples the molecular ion contains the intact 1,3-thiazine. Subsequent ring fragmentation follows a similar pattern within a given series of compounds. 

Covers and uses several fragment types such as augmented atom, atom sequence, atom pair, and ring fusion among others. 

Fischer and Zollinger have studied the reaction of p-chlorobenzenediazonium chloride with 2-naphthol-l-sulfonate. A very interesting dependence of the reaction products and reaction rate on the solvent was observed The initiaUy formed complex 102 decomposes under ring fragmentation in aqueous base (see (44) in Sect. 2.2.3) it does not react in water, ethanol or dimethylsulfoxide, reacts very slowly in acetone or glacial acetic acid to give a small amount of azo compound 7dJ, while in dichloromethane, chloroform, benzene or pyridine quantitative, though slow, formation of the azo compoimd is observed. Ba on kinetic measurements an Sisil-type mechanism was postulated for the formation of the azo dye 103 from the CT-complex 102... 

In contrast, a considerable body of evidence supports the existence of highly polarized intermediates in ring fragmentations of cyclopropane derivatives of type IX. ... 

The presence of a 9,19-cyclopropane ring or a 4-methylene group gives rise to other fragmentation types, fragmentation (a) being greatly... 

Enynes of the structural type (57) have been used as key intermediates in the total synthesis of vitamin D. The enyne (57) can be obtained by a simple synthesis based on Pd-catalyzed coupling of the keto-enol triflate (56) and an alkynic compound containing the vitamin D A-ring fragment (Scheme 38). ... 

Our previous work has established techniques for the generation of four different types of low-coordination phosphorus species by ring fragmentation [some contributions made by other laboratories are summarized in a recent book (5)]. 

We have proceeded to apply the ring fragmentation approach to the generation of yet another type of previously unknown 3-coordinate phosphoryl derivative, where the methylene(oxo)phosphorane has an alkoxy substituent on phosphorus. Such structures (26-29) can be considered as anhydrides of monoalkyl alkylphosphonates. The precursors for these new species have the structures 22-25 (isomer mixtures) they are known compounds (72) from our previous work and are synthesized by the cycloaddition reaction of dihydrophosphinines with dimethyl acetylenedicarboxylate, as used in Scheme 4. 

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