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<a href=http://cialis.boats>cheap cialis no prescription</a> In some embodiments, the present invention provides a compound of formula I, II, III, IV, V, VI, VII, VIII, IX, X, XI, XII, XIII, XIV, or XV, wherein R 6 is hydrogen, thereby forming a compound of formula I a, II a, III a, IV a, V a, VI a, VII a, VIII a, IX a, X a XI a, XII a, XIII a, XIV a, or XV a respectively