Proton, geminate

For instance, the COSY interactions between the proton geminal to the oxygen at 8 5.10 (attached to the methine carbon at 8 72.2) with the protons at 8 2.20 and 2.70 (methylenic protons at the C-4... 

Systematic NMR studies of oxetanes are rather rare. Diastereomeric dimethyloxetanes have been investigated473 and it was found that in the 2,3-dimethyloxetanes the chemical shifts of the protons geminal to the methyl groups are most indicative of the stereochemistry. The 3 values are smaller if they are cis to the respective vicinal methyl group. 

In dithioacetals the proton geminal to the sulfur atoms can be abstracted at low temperature with bases such as Bu"Li. Lithium ion complexing bases such as DABCO, HMPA and TMEDA enhance the process. The resulting anion is a masked acyl carbanion, which enables an assortment of synthetic sequences to be realized via reaction with electrophiles. Thus, a dithioacetal derived from an aldehyde can be further functionalized at the aldehyde carbon with an alkyl halide, followed by thioacetal cleavage to produce a ketone. Dithiane carbanions allow the assemblage of polyfunctional systems in ways complementary to traditional synthetic routes. For instance, the p-hydtoxy ketone systems, conventionally obtained by an aldol process, can now be constructed from different sets of carbon groups. ... 

This process could even be accelerated catalytically. If there is enzyme involvement in production of the carbonyl from the geminal diol, then these TSMs could be considered "suicide" substrates. Another alternative explanation is a "normal" SN2 type reaction between a serine anion and a protonated geminal diol with water as the leaving group. These alternative pathways are not mutually exclusive, but additional work on the kinetics and structural biochemistry of the interaction will be needed to indicate the predominant pathway. 

In 71 there is a multiplet, assigned to the proton geminal to the acetoxy group, at S 4.97 = 24 Hz) whereas in 0-acetyldihydro-... 

The point of reference used in developing the relative configuration of the alkaloids was the hydroxyl group of lycocernuine. In the NMR spectrum of (9-acetyllycocernuine (82) the proton geminal to the... 

The spiro-oxirane ring, in many cases, is replaceci by a CH2OR and an a-hydroxy group. In such cases, the C-18 protons are shifted from ca 2.7 and 3.10 to ca 8 4.00 and 4.50 as observed in 62-65 [35-37 while the C-3 proton, geminal to an acetyl group, appears around 6 5.20-5.4C. Table 6 gives the H and NMR data of 62-65. 

---