Ring conformations of sugars — where do “chair” and “boat” occur? In carbohydrate stereochemistry, the classic chair and boat conformations are characteristic of which class of sugar rings?

Introduction / Context:
Six-membered rings (cyclohexane derivatives) adopt chair and boat conformations to minimize steric strain. Many monosaccharides cyclize to form pyranose rings, which inherit this conformational landscape. Distinguishing which ring sizes show chair/boat forms is key to understanding sugar reactivity and enzyme recognition.

Given Data / Assumptions:

• Pyranoses are six-membered rings; furanoses are five-membered.
• Chair conformations minimize 1,3-diaxial interactions; boats are higher energy.
• Hydroxyl groups can occupy axial or equatorial positions in chairs.

Concept / Approach:

Apply conformational analysis from cyclohexane chemistry to sugar rings. Pyranoses like D-glucose can flip between chairs and, less commonly, boats or skew boats. Furanoses behave more like cyclopentane, adopting envelope/twist forms rather than chair/boat.

Step-by-Step Solution:

Verification / Alternative check:

Textbook Newman projections and models show D-glucopyranose in the 4C1 chair as the dominant conformer; boat conformers are detectable but higher in energy.

Why Other Options Are Wrong:

Options B and C confuse substituent orientation with ring class. Option D is too narrow; many pyranoses show these conformations. Option E misassigns chair/boat to five-membered rings.

Common Pitfalls:

Equating “chair” exclusively with D-glucose or assuming furanoses share the same conformational set.

Final Answer:

Pyranose sugars (six-membered rings)