Abstract
We recently reported the asymmetric synthesis of the two title compounds without the configurational assignments of the newly formed chiral spirocarbons. We now wish to report that both compounds have a (R)-configuration at the spirocarbon based on 1D and 2D nuclear Overhauser enhancement (nOe) experiments.
Highlights
For the past few years we have studied the diastereoselective spiroannulation of simple phenols [111], and we recently reported the asymmetric synthesis of two new spirolactones (+)-1 and (+)-2 (Figure 1) from optically active (S)-3-nitrotyrosine [1]
We wish to report the absolute configuration of these two compounds as determined using one- and twodimensional nuclear Overhauser enhancement [nOe] NMR methods
In the absence of crystals suitable for X-ray analysis, we felt that nOe techniques would be the best way to determine these
Summary
For the past few years we have studied the diastereoselective spiroannulation of simple phenols [111], and we recently reported the asymmetric synthesis of two new spirolactones (+)-1 and (+)-2 (Figure 1) from optically active (S)-3-nitrotyrosine [1]. Since carbon 3 in the lactone ring has a (S)-configuration as shown in Figure 1 [12], irradiation of H3 should affect only one of H10 (structure A) or H6 (structure B) assuming that these protons are in close enough proximity to H3 to be affected. It is normally assumed that 1H nOe can be observed between protons located within 500 pm (5Ǻ) of each other [13,14]. This is about twice the distance separating 1,3-diaxial protons on the chair form of cyclohexane (~2.6Ǻ) [14]. When comparing models of cyclohexane with either structures A or B, we estimated that the distance between H3 and either H10 or H6 falls within the range normally expected to observe nOe [15]
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