Base-induced isomerization of red uroleuconaphins revisited: characterization and absolute stereochemistry of the yellow aphid pigments uroleuconaphins A₂ and B₂

Abstract

We present herein the base-induced isomerization of red uroleuconaphins A1 and B1 to yellow uroleuconaphins A2 and B2, respectively. Based on our preliminary results, we optimized the conversion conditions by...

Silica gel-mediated chemical degradation of dimeric pyranonaphthoquinones into their monomeric units

“…In contrast, the reaction using 5a , the C(10a)–α–H diastereomer, was sluggish to complete the degradation for five days because of the significantly slower enolization at C10a. 6 As in the control experiments, the reaction did not work well without silica gel or MeOH. Performing the reaction in aqueous methanol gave the oxidized product 8 in 46% yield.…”

“…Under basic conditions, the red pigment 1 was isomerized through acetalization to uroleuconaphin A 2 ( 3 ), 5 a yellow pigment. 6 In contrast, the exposure of 1 to strong acids led to dehydrative cyclizations to produce viridaphin A 2 ( 2 ), 7 a green pigment (Scheme 1). 8…”

“…Under basic conditions, the red pigment 1 was isomerized through acetalization to uroleuconaphin A 2 (3), 5 a yellow pigment. 6 In contrast, the exposure of 1 to strong acids led to dehydrative cyclizations to produce viridaphin A 2 (2), 7 a green pigment (Scheme 1). 8 During this study, we observed that yellow pigment 3 gradually converted into 1 through the reverse reaction by exposure to silica gel in MeOH.…”

“…8 During this study, we observed that yellow pigment 3 gradually converted into 1 through the reverse reaction by exposure to silica gel in MeOH. To suppress the isomerization, two non-hydrogen-bonded phenols at C7 and C7 0 in 3 were protected as methyl groups by treatment with TMSCHN 2 to give 4, 6,9 which was stable under exposure to silica gel at room temperature (Scheme 2). Further investigations revealed that the chemical degradation of 4 proceeded to give the constituting monomeric compounds by treatment with silica gel at elevated temperatures.…”