Spectral properties of four monohemiaminals belonging to the thiaspirane class of nuphar alkaloids are compared and employed in the structure elucidation of three of these compounds. The new monohemiaminals are 6′-hydroxythiobinupharidine and 6-hydroxythionuphlutine B, both isolated from N. luteum, and 6′-hydroxythionuphlutine B, prepared from 6,6′-dihydroxythionuphlutine B. The nmr showed that the hemiaminal group in each of the three monohemiaminals was located at one of two C-6 positions. Distinction of a C-6 from a C-6′ hemiaminal was made chiefly by (1) deuteride reduction to singly labeled thiaspirane followed by a mass spectral analysis for the extent of mfe 178 to 179 shift; and (2) the CD of the monohemiaminals in acid solution. Singly deuterated thiaspiranes which were labeled at C-6 resulted in mfe 178 shifting to 179 by more than 90%. In contrast, thiaspiranes singly labeled at C-6′ resulted in only a 10% shift of mfe 178 to 179. The CD of all the C-6′ hemiaminals in acid solution showed positive CD bands in the region of 260-280 nm but the CD of 6-hydroxythiobinupharidine and 6-hydroxythionuphlutine B show positive and negative CD bands, respectively, in the 290-310-nm region. © 1974, American Chemical Society. All rights reserved.