Cyclic acetals, rearrangement

Methylenetriphenyl phosphorane has been used for the synthesis of a variety of cyclic compounds by a combination of ring closure and nucleophilic ylid reaction A new simple synthesis of isoflavones via a thallium(III) acetate-promoted rearrangement of chalcones has been published ... 

In contrast to the acyclic aUyl silyl ketene acetals, cyclic substrates may preferentially rearrange through either the chair or boat transitions states. In 1981 Bartlett and Pizzo reported that treatment of cyclohexenyl propionates under either set of conditions reported by Ireland resulted in the formation of the same major isomer (Scheme 4.17) [20]. They concluded that the -silyl ketene acetal rearranged preferentially via a chair-hke transition state, while the Z-silyl ketene acetal rearranged via a boat-like transition state. These conclusions were recently supported computationally by Houk et al., who reported a 1.0 kcal/mol preference for the boat transition state for the Z-geometry in the analogous OMe ketene acetal and a 1.4 kcal/mol preference for the chair transition state for the -geometry in the OMe ketene acetal [18]. 

Titanium tetrachloride 3-Hydroxyethers from acetals by rearrangement s. 17, 790 o-Phenolketones from phenolesters Fries rearrangement s. 19, 798 Poly phosphoric acid Cyclic ketocarboxylic acids from dicarboxylic acid anhydrides s, 19,961... 

Perkin condensation a condensation of an aromatic aldehyde and acetic acid, pericyclic reaction a concerted reaction that takes place as the result of a cyclic rearrangement of electrons. 

Miscellaneous Reactions. DMF acetals catalyze rearrangement reactions of allylic alcohols to p,7-unsaturated amides. This reaction, which involves a [2,3]-sigmatropic rearrangement, occurs with complete transfer of chirality. Thus the reaction of the (/ ,Z)-allylic alcohol (eq 15) with DMF dimethyl acetal gives the enantiomerically pure (R, )-p,7-unsaturated amide as the only product. The (5, )-isomer also rearranges mainly to the (/ , )-amide, with only a trace of the (5,Z)-isomer. It has been suggested that both rearrangements proceed via a five-membered cyclic transition state with a carbene-like function. ... 

Halogenation of the 7 position also proves compatible with good antiinflammatory activity. Construction of this compound, aclomethasone dipropionate (80), starts by introduction of the required unsaturation at the 6,7 position by dehydrogenation with DDQ (76). The highly hindered nature of the hydroxyl at position 17 requires that a roundabout scheme be used for formation of the corresponding ester. Thus treatment of 76 with ethyl orthoformate affords first the cyclic orthoformate This then rearranges to the 17 ester on exposure to acetic acid. Acylation of the 21 alcohol is accomplished in straightforward fashion with... 

Jasmonates are important odorant compounds. For the synthesis of new substances of this type, Giersch and Forris developed a domino Claisen/ene/retro-ene process which allows the acid-catalyzed transformation of sorbyl alcohol 4-285 and the cyclic acetals 4-286 into the cycloalkenone 4-292 with the proposed intermediates 4-287 to 4-291 (Scheme 4.61) [97]. A similar domino process had been described by Srikrishna and coworkers [98]. Acyclic acetals gave only the Claisen rearrangement products. 

A methylenation of cyclic carbonates such as 6/4-132 using dimethyltitanocene to give a ketene acetal, followed by a subsequent Claisen rearrangement, allowed the synthesis of medium-ring lactones such as 6/4-133 in good yields these are otherwise difficult to obtain. In this transformation, 6/4-133 is formed as a l l-mix-ture of the two atropisomers 6/4-133a and 6/4-133b (Scheme 6/4.33). The substrate... 

Other non-traditional preparations of 1,2,3-triazoles have been reported. The rearrangement in dioxane/water of (Z)-arylhydrazones of 5-amino-3-benzoyl-l,2,4-oxadiazole into (2-aryl-5-phenyl-27/-l,2,3-triazol-4-yl)ureas was investigated mechanistically in terms of substituents on different pathways <06JOC5616>. A general and efficient method for the preparation of 2,4-diary 1-1,2,3-triazoles 140 from a-hydroxyacetophenones 139 and arylhydrazines is reported <06SC2461>. 5-Alkylamino-] //-], 2,3-triazoles were obtained by base-mediated cleavage of cycloadducts of azides to cyclic ketene acetals <06S1943>. Oxidation of N-... 

Second, one should take into account the stability of nitroso acetals (343) associated with the possibility of their rearrangement to give oxazines (344). In addition, it should be taken into account that cyclic nitroso acetals (343) can be transformed into halo derivatives of dihydrooxazines (345). (These transformations of nitroso acetals (343) will be considered in more detail in Section 3.5.4.3.3.)... 

Interestingly, Zn(OTf)2 is of little use for the rearrangement of cyclic six-membered N -siloxynitroso acetals (507a-g) into the corresponding 5,6-dihydro-4H -oxazines (508a-g) (Scheme 3.263, Table 3.35) (264, 535). 

Table 3.35 Rearrangement of cyclic six-membered N -siloxynitroso acetals 507a-g.

Only two general methods have been developed for the synthesis of the macrocyclic annulenes.9 The first of these, developed by Sondheimer and co-workers, involves the oxidative coupling of a suitable terminal diacetylene to a macrocyclic polyacetylene of required ring size, using typically cupric acetate in pyridine. The cyclic compound is then transformed to a dehydroannulene, usually by prototropic rearrangement effected by potassium i-butoxide. Finally, partial catalytic hydrogenation of the triple bonds to double bonds leads to the annulene. 

FORMATION AND PHOTOCHEMICAL WOLFF REARRANGEMENT OF CYCLIC a-DIAZO KETONES D-NORANDROST-5-EN-3 -0L-16-CARB0XYLIC ACIDS, 52, 53 FORMIC ACID, AZIDO—, tert-BUTYL ESTER, 50, 9 Formylation, with acetic formic anhydride, 50, 2 p-FORMYLBENZENESULFONAMIDE, ... 

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

---