Medium ring lactones

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... 

In the presence of proper substituents the primarily formed diradical rearranges to a more stable one, as by a ring opening reaction. In (2.7) medium ring lactones are synthesized by such a sequence 2U). 

Medium ring lactones are increasingly important since they are contained in an ever-growing number of natural products (Rousseau, 1995 Collins, 1999), and their preparation becomes of great interest. 

This iodine(in) oxidative fragmentation has been exploited to generate unsaturated medium ring lactones in good yields and with complete control of double bond stereochemistiy as shown in Scheme... 

A new method for the synthesis of medium-ring lactones from cyclic sulphide precursors has been reported. The thiolo-lactone (31) rearranges... 

Masamune has made use of this property for synthesis of medium ring lactones (macrolides), for example, zearalenone dimethyl ether ethylene ketal (5) from the hydroxy ester (4). On treatment with mercuric trifluoroacetate at 25°, (4) is converted within 5 min. into the lactone (5) in >90% yield (recrystalhzed). 

Speckamp et al. have reported the synthesis of medium-ring lactones by Cu(bipy)Cl-catalyzed cyclization of alkenyl trichloroacetates. In the case of 5-hexenyl trichloroacetate (175), a 0.06 M solution in a sealed tube at 190°C gave the trichlorolactone (176) in fair yield (Equation (15))... 

The preparation, reactivity and applications of medium-ring lactones have been comprehensively reviewed by Rousseau <95T2777>. This section will therefore concentrate on the chemistry of other nine-membered oxacycles. 

Fragmentation of Lactols to Unsaturated Medium-ring Lactones. Ring cleavage to form a medium-sized ring lactone with a transannular double bond has been observed (eq 21). ... 

Simonot B, Rousseau G (1993) Preparation of 7-membered and medium-ring lactones by iodo lactonization. J Org Chem 58 4—5... 

New General Synthesis of medium-ring lactones via a Regioselective -Fragmentation of Alkoxyl Radicals Generated from Catacondensed Lactols... 

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