Spring Boot Java 6 java.lang.UnsupportedClassVersionError: X: Unsupported major.minor 51.0

I need to put an application built on top of Spring Boot to run in an environment with Java 6, I am finding some problems. I did some testing, I loaded a new project with start.spring.io installed on Java 6 and the problem still persists.

I find it strange because the project rebuilt with start.spring.io (installed in Java 6) does not apply the recommended settings in the documentation to run in Java 6

After applying the recommended settings described on the site, the error persists.

With Jetty settings:

ERROR 7776 --- [main] osboot.SpringApplication: Application startup failed

java.lang.UnsupportedClassVersionError: org / apache / tomcat / jdbc / pool / DataSource: Unsupported major.minor version 51.0

      

        
        
        
      

    

With Tomcat configurations:

 05/14/2015 07: 50: 18,112 ERROR 10852 --- [main]
 osboot.SpringApplication: Application startup failed

 Exception in thread "main" java.lang.UnsupportedClassVersionError:
 javax / annotation / ManagedBean: Unsupported major.minor version 51.0

      

        
        
        
      

    

JDK version: 1.6.0_45

Spring Boot: v1.2.3.RELEASE

Build with Gradle

Any suggestions on what I might be doing wrong?