Furofuranes

Similarly, furofuranes and benzofurofuranes known as substructure of afla-toxins are obtainable via hydroformylation of unsaturated diols. A representative example is given in Scheme 5 [34]. 

A further remarkable finding in the hydrolysis of aflatoxin B1 exo-8,9-epoxide is the relative instability of the dihydrodiol, which under basic conditions exists in equilibrium with an aflatoxin dialdehyde, more precisely a furofuran-ring-opened oxy anionic a-hydroxy dialdehyde (10.134, Fig. 10.30). The dihydrodiol is the predominant or exclusive species at pH < 7, whereas this is true for the dialdehyde at pH >9, the pK value of the equilibrium being 8.2 [204], The dialdehyde is known to form Schiff bases with primary amino groups leading to protein adducts. However, the slow rate of dialdehyde formation at physiological pH and its reduction by rat and human aldo-keto reductases cast doubts on the toxicological relevance of this pathway [206]. 

Upon supplementing the first alkylation by an aldol reaction, other types of a-hydroxylated lignans should be attainable as well. In the case in which both C-C bond formations are aldol reactions, furofuran lignans, such as paulownin or analogs, should be obtained. On the other hand, aldol addition... 

For 2,3-dihydrofuran itself no molecular dimensions have been recorded as yet, though a few natural products containing the system have been studied by X-ray diffraction. These include prerotundifolin (73CL1035) and also nepetaefolin (75JA6236), some relevant dimensions of which are shown (68). A c/s-fused furofuran system is characteristic of several... 

Table 23 13C NMR Parameters for the Furofuran Segment of Sterigmatocystin <74TL449i,...

The fully aromatic A,B-diheteropentalenes do not occur in nature. However, the reduced furofurans and pyrrolopyrroles are present in a variety of natural products such as lignans, fungal metabolites and alkaloids. It is beyond the scope of this chapter to present an exhaustive survey of these compounds. However, a few selected examples are presented in formulae (376)-(380). 

Dihydro-l,3-dioxepin 47b reacted with dimethyl diazomalonate in a double [3+2] cycloaddition reaction to give furofuran derivative 54 as a mixture of stereoisomers (Scheme 4) <2000HCA966>. 

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