United States v. Rene Lugo-Barcenas, 57 F.4th 633

Rene Lugo-Barcenas pleaded guilty to conspiracy to distribute at least 50 grams of a methamphetamine-containing substance and agreed to a plea with an appeal waiver that preserved only claims of an "illegal sentence."
At sentencing parties disputed the drug quantity; defense argued methamphetamine Guidelines are unduly harsh and create unwarranted disparities.
The district court applied the Sentencing Guidelines, calculated a 168–210 month range, and sentenced Lugo-Barcenas to 168 months.
The government moved to enforce the appeal waiver to bar most of Lugo-Barcenas's challenges to his sentence.
Lugo-Barcenas raised a new, unpreserved Fifth Amendment equal-protection challenge to the Guidelines’ purity-based treatment of methamphetamine (pure vs. mixture).
The Eighth Circuit held the waiver barred most claims, allowed the constitutional claim to proceed, reviewed it for plain error, and rejected it; the remainder of the appeal was dismissed.

Issue	Plaintiff's Argument	Defendant's Argument	Held
Whether the plea-waiver bars challenge to Guidelines being unduly harsh/unwarranted disparities	Lugo-Barcenas: Guidelines for methamphetamine are too harsh and create unwarranted disparities; sentence unreasonable	Government: Waiver expressly bars appeals of misapplication/unreasonableness of Guidelines	Waiver bars this argument — appeal dismissed on this ground
Whether the waiver bars a Fifth Amendment equal-protection challenge to purity-based Guidelines	Lugo-Barcenas: Purity distinction lacks rational basis today and violates Due Process/ equal-protection component	Government: Waiver covers sentencing errors; this argument is barred	Waiver does not clearly bar constitutional challenge; court reviewed it on plain-error and rejected it
Whether the district court treated Guidelines as mandatory/presumptively reasonable	Lugo-Barcenas: Court’s statement that it was "obligat[ed] to apply" Guidelines shows mandatory treatment and constitutional error	Government: This is a garden-variety misapplication of Guidelines covered by waiver	Barred by waiver; argument raised too late as constitutional gloss was in reply brief
Whether the district court failed to adequately consider §3553(a) factors/substantive reasonableness	Lugo-Barcenas: Court inadequately weighed factors; sentence substantively unreasonable	Government: Waiver bars appeals of abuse of discretion/unreasonable sentence	Barred by waiver — dismissed

Guice, 925 F.3d 990 (8th Cir. 2019) (government must prove plea waiver clear and unambiguous)
Bradford, 806 F.3d 1151 (8th Cir. 2015) (similar plea-waiver language may leave open constitutional sentencing claims)
Valencia, 829 F.3d 1007 (8th Cir. 2016) (ordinary definition of "illegal sentence" is one exceeding statutory maximum)
Fisher, 25 F.4th 1080 (8th Cir. 2022) (plain-error review for arguments not presented below)
Darden, 915 F.3d 579 (8th Cir. 2019) (issues raised first in reply brief generally not considered)
Bolling v. Sharpe, 347 U.S. 497 (1954) (Due Process Clause of Fifth Amendment encompasses equal-protection component)
Cuthbert, 419 F. Supp. 3d 1265 (D. Idaho 2019) (example explaining purity-based quantity equivalence in Guidelines)