%0 Journal Article
%T Formation of 5-Hydroxy-3-methoxy-1,4-naphthoquinone and 8-Hydroxy-4-methoxy-1,2-naphthoquinone from Juglone
%A Bastian Blauenburg
%A Mikko Mets£¿-Ketel£¿
%A Karel D. Klika
%J ISRN Organic Chemistry
%D 2012
%R 10.5402/2012/274980
%X From the treatment of 5-hydroxy-1,4-naphthoquinone (juglone) with acetic anhydride and H2SO4 followed subsequently by treatment with methanolic HCl, 5-hydroxy-3-methoxy-1,4-naphthoquinone (3-methoxy juglone) and 8-hydroxy-4-methoxy-1,2-naphthoquinone were obtained as products rather than the anticipated product 2,5-dihydroxy-1,4-naphthoquinone (2-hydroxy juglone). The reaction and the identification of the products are discussed in terms of NMR and DFT calculations. 1. Introduction As part of our ongoing investigation [1] into the C-ribosylation of (R)-prealnumycin (1) by AlnA, an enzyme produced by Streptomyces albus (Scheme 1), it was decided to test various 1,4-naphthoquinone substrates against AlnA in order to identify the structural motifs necessary for, or inhibitory to, reaction. Scheme 1: The C-ribosylation of ( R)-prealnumycin ( 1) by Streptomyces albus. Thus, we considered 2,5-dihydroxy-1,4-naphthoquinone (2, 2-hydroxy juglone, Figure 1) as a test substrate since it could potentially reveal aspects of the charge required to be present on C-3 for the reaction to proceed. Additionally, the facile preparation of 2,5-dihydroxy-7-methyl-1,4-naphthoquinone (2-hydroxy-7-methyl juglone, 3) from 5-hydroxy-7-methyl-1,4-naphthoquinone (7-methyl juglone, 4) had been reported [2] by treatment first with acetic anhydride and H2SO4 and then subsequently with methanolic HCl. After treating 5-hydroxy-1,4-naphthoquinone (5, juglone) as prescribed, we obtained not 2-hydroxy juglone (2) as anticipated, but rather 5-hydroxy-3-methoxy-1,4-naphthoquinone (6, 3-methoxy juglone) and 8-hydroxy-4-methoxy-1,2-naphthoquinone (7). Mahapatra et al. [2], in turn, seem to have taken their preparation from Lillie and Musgrave [3] who performed precisely the same reaction. Interestingly, whilst the former workers only reported the one product, namely, 2-hydroxy-7-methyl juglone (3), the latter pair reported the formation of both 3 (as the major product) and its regioisomer 3,5-dihydroxy-7-methyl-1,4-naphthoquinone (3-hydroxy-7-methyl juglone, 8). Mahapatra et al. [2] only reported 1H NMR chemical shifts ( ) for their compound, and indeed only one OH signal¡ªwhich they assigned as HO-5¡ªwith no carbon or correlation spectra reported. As best we can ascertain, it appears that Mahapatra et al. [2] based their structural assignment on the work of Lillie and Musgrave [3] and the of HO-5 (vide infra). This structural assignment therefore carries with it some concern, not because it is a labile proton per se, but rather because of its value in relation to the reported value [3]. The
%U http://www.hindawi.com/journals/isrn.organic.chemistry/2012/274980/